SAPI: Move core.get_us_time() to Util module

src/script/lua_api/l_env.h

@@ -168,9 +168,6 @@ class ModApiEnvMod : public ModApiBase {
 	// stops forceloading a position
 	static int l_forceload_free_block(lua_State *L);
 
-	// get us precision time
-	static int l_get_us_time(lua_State *L);
-
 public:
 	static void Initialize(lua_State *L, int top);
 };