kernel/thread: Remove global GetCurrentThread()

This is only used in one place, so we can fold it into the calling code,
eliminating a place for the global system instance to be used.

src/core/hle/kernel/thread.h

@@ -680,9 +680,4 @@ private:
     std::string name;
 };
 
-/**
- * Gets the current thread
- */
-Thread* GetCurrentThread();
-
 } // namespace Kernel