Force prefabs off for Editor smoke test

Signed-off-by: nvsickle <nvsickle@amazon.com>

AutomatedTesting/Gem/PythonTests/smoke/test_Editor_NewExistingLevels_Works.py

@@ -28,4 +28,4 @@ class TestAutomation(TestAutomationBase):
 
         from . import Editor_NewExistingLevels_Works as test_module
 
-        self._run_test(request, workspace, editor, test_module)
+        self._run_test(request, workspace, editor, test_module, extra_cmdline_args=["--regset=/Amazon/Preferences/EnablePrefabSystem=false"])