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")