Codechange: remove _realtime_tick variable

(cherry picked from commit fe451b8dc7)

# Conflicts:
#	src/debug.cpp
#	src/debug.h
#	src/video/video_driver.cpp

src/debug.h

@@ -131,9 +131,6 @@ void CDECL ShowInfoF(const char *str, ...) WARN_FORMAT(1, 2);
 
 const char *GetLogPrefix();
 
-/** The real time in the game. */
-extern uint32 _realtime_tick;
-
 void ClearDesyncMsgLog();
 void LogDesyncMsg(std::string msg);
 char *DumpDesyncMsgLog(char *buffer, const char *last);