Cleaning up com.android.window.flags.supports_multi_instance_system_ui flag

NO_EXPORTED_FLAG_DELETION_CHECK=Flag was not used internally

Bug: 409607488
Bug: 329708298
Flag: EXEMPT cleanup
Test: m droid

Change-Id: Ifb9f7cea6bc2eadc9722d41357c83f58164b62f5
Signed-off-by: Winson Chung <winsonc@google.com>