Remove TaskGraph::Drain which was only added initially for testing

The drain function was used only before the API gained the ability to
wait on the completion of a graph. This is the correct way to "drain"
the task executor of work.

Signed-off-by: Jeremy Ong <jcong@amazon.com>

Code/Framework/AzCore/AzCore/Task/TaskExecutor.cpp

@@ -355,24 +355,8 @@ namespace AZ
         m_workers[++m_lastSubmission % m_threadCount].Enqueue(&task);
     }
 
-    void TaskExecutor::Drain()
-    {
-        m_isDraining = true;
-        if (m_graphsRemaining == 0)
-        {
-            return;
-        }
-        m_drainSemaphore.acquire();
-    }
-
     void TaskExecutor::ReleaseGraph()
     {
-        uint64_t graphsRemaining = --m_graphsRemaining;
-
-        if (graphsRemaining == 0 && m_isDraining)
-        {
-            m_drainSemaphore.release();
-            m_isDraining = false;
-        }
+        --m_graphsRemaining;
     }
 } // namespace AZ

Code/Framework/AzCore/AzCore/Task/TaskExecutor.h

@@ -76,9 +76,6 @@ namespace AZ
 
         void Submit(Internal::Task& task);
 
-        // Wait until tasks are cleared from the executor (note, does not prevent future tasks from being submitted)
-        // If this is used, it's expected to be used between frames to shutdown the engine
-        void Drain();
     private:
         friend class Internal::TaskWorker;
 
@@ -88,7 +85,5 @@ namespace AZ
         uint32_t m_threadCount = 0;
         AZStd::atomic<uint32_t> m_lastSubmission;
         AZStd::atomic<uint64_t> m_graphsRemaining;
-        AZStd::atomic<bool> m_isDraining;
-        AZStd::binary_semaphore m_drainSemaphore;
     };
 } // namespace AZ

Code/Framework/AzCore/Tests/TaskTests.cpp

@@ -543,95 +543,6 @@ namespace UnitTest
 
         EXPECT_EQ(3 | 0b100000, x);
     }
-
-    TEST_F(TaskGraphTestFixture, ExecutorDrainRetained)
-    {
-        bool drainDone = false;
-        AZStd::binary_semaphore taskStart;
-        AZStd::binary_semaphore threadLaunched;
-        AZStd::binary_semaphore threadFinished;
-
-        TaskGraph graph;
-        auto a = graph.AddTask(
-            defaultTD,
-            [&]
-            {
-                taskStart.acquire();
-            });
-
-        graph.SubmitOnExecutor(*m_executor);
-
-        AZStd::thread drainThread{ [this, &drainDone, &threadLaunched, &threadFinished]
-                                   {
-                                       threadLaunched.release();
-                                       m_executor->Drain();
-                                       drainDone = true;
-                                       threadFinished.release();
-                                   } };
-
-
-        // Wait until our drain thread has launched
-        threadLaunched.acquire();
-
-        // The task itself hasn't started, so the drain should still be blocking
-        EXPECT_EQ(false, drainDone);
-
-        // Allow the task to finish
-        taskStart.release();
-
-        // Wait for the drain thread to wrap up
-        threadFinished.acquire();
-
-        // We successfully drained the executor
-        EXPECT_EQ(true, drainDone);
-
-        drainThread.join();
-    }
-
-    TEST_F(TaskGraphTestFixture, ExecutorDrainDetached)
-    {
-        bool drainDone = false;
-        AZStd::binary_semaphore taskStart;
-        AZStd::binary_semaphore threadLaunched;
-        AZStd::binary_semaphore threadFinished;
-
-        TaskGraph graph;
-        auto a = graph.AddTask(
-            defaultTD,
-            [&]
-            {
-                taskStart.acquire();
-            });
-        graph.Detach();
-
-        graph.SubmitOnExecutor(*m_executor);
-
-        AZStd::thread drainThread{ [this, &drainDone, &threadLaunched, &threadFinished]
-                                   {
-                                       threadLaunched.release();
-                                       m_executor->Drain();
-                                       drainDone = true;
-                                       threadFinished.release();
-                                   } };
-
-
-        // Wait until our drain thread has launched
-        threadLaunched.acquire();
-
-        // The task itself hasn't started, so the drain should still be blocking
-        EXPECT_EQ(false, drainDone);
-
-        // Allow the task to finish
-        taskStart.release();
-
-        // Wait for the drain thread to wrap up
-        threadFinished.acquire();
-
-        // We successfully drained the executor
-        EXPECT_EQ(true, drainDone);
-
-        drainThread.join();
-    }
 } // namespace UnitTest
 
 #if defined(HAVE_BENCHMARK)