Removed the vi format comments from the source

Vim users can use the [editorconfig plugin](https://github.com/editorconfig/editorconfig-vim) to automatically set tab spacing for the SDL coding style.

Fixes https://github.com/libsdl-org/SDL/issues/6903

src/thread/stdcpp/SDL_sysmutex.cpp

@@ -97,5 +97,3 @@ SDL_UnlockMutex(SDL_mutex *mutex) SDL_NO_THREAD_SAFETY_ANALYSIS /* clang doesn't
     mutex->cpp_mutex.unlock();
     return 0;
 }
-
-/* vi: set ts=4 sw=4 expandtab: */