OS: Remove unused get_splash_tick_msec

It was added in 3e20391bf6 but it doesn't seem
particularly useful, and it was only implemented for the custom splash branch
and not the default one, so it could return an uninitialized int.

core/os/os.cpp

@@ -80,10 +80,6 @@ String OS::get_iso_date_time(bool local) const {
 		   timezone;
 }
 
-uint64_t OS::get_splash_tick_msec() const {
-	return _msec_splash;
-}
-
 double OS::get_unix_time() const {
 	return 0;
 }