kernel/thread: Remove unimplemented function prototype

Given there's no implementation, we may as well remove the code
entirely.

src/core/hle/kernel/thread.h

@@ -270,12 +270,6 @@ Thread* GetCurrentThread();
  */
 void WaitCurrentThread_Sleep();
 
-/**
- * Waits the current thread from an ArbitrateAddress call
- * @param wait_address Arbitration address used to resume from wait
- */
-void WaitCurrentThread_ArbitrateAddress(VAddr wait_address);
-
 /**
  * Stops the current thread and removes it from the thread_list
  */