Remove references to unsupported "xlib" windowing system support (#4028)

Support for X clients is provided by xcb instead.

Signed-off-by: Chris Burel <burelc@amazon.com>

Code/Framework/AzFramework/Platform/Linux/AzFramework/Application/Application_Linux.cpp

@@ -21,9 +21,6 @@ namespace AzFramework
 #elif PAL_TRAIT_LINUX_WINDOW_MANAGER_WAYLAND
         #error "Linux Window Manager Wayland not supported."
         return nullptr;
-#elif PAL_TRAIT_LINUX_WINDOW_MANAGER_XLIB
-        #error "Linux Window Manager XLIB not supported."
-        return nullptr;
 #else
         #error "Linux Window Manager not recognized."
         return nullptr;

Code/Framework/AzFramework/Platform/Linux/AzFramework/Windowing/NativeWindow_Linux.cpp

@@ -17,9 +17,6 @@ namespace AzFramework
 #elif PAL_TRAIT_LINUX_WINDOW_MANAGER_WAYLAND
         #error "Linux Window Manager Wayland not supported."
         return nullptr;
-#elif PAL_TRAIT_LINUX_WINDOW_MANAGER_XLIB
-        #error "Linux Window Manager XLIB not supported."
-        return nullptr;
 #else
         #error "Linux Window Manager not recognized."
         return nullptr;

Code/Framework/AzFramework/Platform/Linux/platform_linux.cmake

@@ -7,7 +7,7 @@
 #
 
 # Based on the linux window manager trait, perform the appropriate additional build configurations
-# Only 'xcb', 'wayland', and 'xlib' are recognized
+# Only 'xcb' and 'wayland' are recognized
 if (${PAL_TRAIT_LINUX_WINDOW_MANAGER} STREQUAL "xcb")
 
     find_library(XCB_LIBRARY xcb)
@@ -23,10 +23,6 @@ elseif(PAL_TRAIT_LINUX_WINDOW_MANAGER STREQUAL "wayland")
 
     set(LY_COMPILE_DEFINITIONS PUBLIC PAL_TRAIT_LINUX_WINDOW_MANAGER_WAYLAND)
 
-elseif(PAL_TRAIT_LINUX_WINDOW_MANAGER STREQUAL "xlib")
-
-    set(LY_COMPILE_DEFINITIONS PUBLIC PAL_TRAIT_LINUX_WINDOW_MANAGER_XLIB)
-
 else()
 
     message(FATAL_ERROR, "Linux Window Manager ${PAL_TRAIT_LINUX_WINDOW_MANAGER} is not recognized")

Gems/Atom/RHI/Vulkan/3rdParty/Platform/Linux/glad_vulkan_linux.cmake

@@ -14,10 +14,6 @@ elseif(PAL_TRAIT_LINUX_WINDOW_MANAGER STREQUAL "wayland")
     set(GLAD_VULKAN_COMPILE_DEFINITIONS 
         VK_USE_PLATFORM_WAYLAND_KHR
     )
-elseif(PAL_TRAIT_LINUX_WINDOW_MANAGER STREQUAL "xlib")
-    set(GLAD_VULKAN_COMPILE_DEFINITIONS 
-        VK_USE_PLATFORM_XLIB_KHR
-    )
 else()
     message(FATAL_ERROR, "Linux Window Manager ${PAL_TRAIT_LINUX_WINDOW_MANAGER} is not recognized")
 endif()

Gems/Atom/RHI/Vulkan/Code/Source/Platform/Linux/RHI/WSISurface_Linux.cpp

@@ -41,9 +41,6 @@ namespace AZ
 #elif PAL_TRAIT_LINUX_WINDOW_MANAGER_WAYLAND
             #error "Linux Window Manager Wayland not supported."
             return RHI::ResultCode::Unimplemented;
-#elif PAL_TRAIT_LINUX_WINDOW_MANAGER_XLIB
-            #error "Linux Window Manager XLIB not supported."
-            return RHI::ResultCode::Unimplemented;
 #else
             #error "Linux Window Manager not recognized."
             return RHI::ResultCode::Unimplemented;

cmake/Platform/Linux/PAL_linux.cmake

@@ -41,5 +41,6 @@ set(LY_ASSET_DEPLOY_ASSET_TYPE "pc" CACHE STRING "Set the asset type for deploym
 ly_set(LY_PYTHON_CMD ${CMAKE_CURRENT_SOURCE_DIR}/python/python.sh)
 
 # Set the default window manager that applications should be using on Linux 
-# Note: Only ("xcb", "wayland", or "xlib" should be considered)
-set(PAL_TRAIT_LINUX_WINDOW_MANAGER "xcb" CACHE STRING "Sets the Window Manager type to use when configuring Linux (xcb, wayland, or xlib)")  
+# Note: Only ("xcb" or "wayland" should be considered)
+set(PAL_TRAIT_LINUX_WINDOW_MANAGER "xcb" CACHE STRING "Sets the Window Manager type to use when configuring Linux")  
+set_property(CACHE PAL_TRAIT_LINUX_WINDOW_MANAGER PROPERTY STRINGS xcb wayland)