common/thread: Remove unused CurrentThreadId()

This is an old function that's no longer necessary. C++11 introduced
proper threading support to the language and a thread ID can be
retrieved via std::this_thread::get_id() if it's ever needed.

src/common/thread.cpp

@@ -25,16 +25,6 @@
 
 namespace Common {
 
-int CurrentThreadId() {
-#ifdef _MSC_VER
-    return GetCurrentThreadId();
-#elif defined __APPLE__
-    return mach_thread_self();
-#else
-    return 0;
-#endif
-}
-
 #ifdef _WIN32
 // Supporting functions
 void SleepCurrentThread(int ms) {

src/common/thread.h

@@ -13,8 +13,6 @@
 
 namespace Common {
 
-int CurrentThreadId();
-
 void SetThreadAffinity(std::thread::native_handle_type thread, u32 mask);
 void SetCurrentThreadAffinity(u32 mask);