Silence Input.vibrate_handheld() warning as it's already documented

The warning causes messages to be spammed if you are calling this
method in a game that runs on both desktop and mobile platforms,
unless you guard all calls to `Input.vibrate_handheld()` with
`OS.has_feature("mobile") or OS.has_feature("web")`.

Since the limitation is already documented (and is obvious enough
given the method's name), the warning message is redundant.

core/os/os.cpp

@@ -155,10 +155,6 @@ int OS::get_process_id() const {
 	return -1;
 }
 
-void OS::vibrate_handheld(int p_duration_ms) {
-	WARN_PRINT("vibrate_handheld() only works with Android, iOS and Web");
-}
-
 bool OS::is_stdout_verbose() const {
 	return _verbose_stdout;
 }