video_core/gpu_thread: Remove unimplemented WaitForIdle function prototype

This function didn't have a definition, so we can remove it to prevent
accidentally attempting to use it.

src/video_core/gpu_thread.h

@@ -113,9 +113,6 @@ public:
     /// Notify rasterizer that any caches of the specified region should be flushed and invalidated
     void FlushAndInvalidateRegion(VAddr addr, u64 size);
 
-    /// Waits the caller until the GPU thread is idle, used for synchronization
-    void WaitForIdle();
-
 private:
     /// Pushes a command to be executed by the GPU thread
     void PushCommand(CommandData&& command_data, bool wait_for_idle, bool allow_on_cpu);