Drop old semaphore implementation

- Removed platform-specific implementations.
- Now all semaphores are in-object, unless they need to be conditionally created.
- Similarly to `Mutex`, provided a dummy implementation for when `NO_THREADS` is defined.
- Similarly to `Mutex`, methods are made `const` for easy use in such contexts.
- Language bindings updated: `wait()` and `post()` are now `void`.
- Language bindings updated: `try_wait()` added.

Bonus:
- Rewritten the `#ifdef` in `mutex.h` to meet the code style.

core/bind/core_bind.cpp

@@ -2576,32 +2576,28 @@ void _Marshalls::_bind_methods() {
 
 ////////////////
 
-Error _Semaphore::wait() {
+void _Semaphore::wait() {
 
-	return semaphore->wait();
+	semaphore.wait();
 }
 
-Error _Semaphore::post() {
+Error _Semaphore::try_wait() {
 
-	return semaphore->post();
+	return semaphore.try_wait() ? OK : ERR_BUSY;
+}
+
+void _Semaphore::post() {
+
+	semaphore.post();
 }
 
 void _Semaphore::_bind_methods() {
 
 	ClassDB::bind_method(D_METHOD("wait"), &_Semaphore::wait);
+	ClassDB::bind_method(D_METHOD("try_wait"), &_Semaphore::try_wait);
 	ClassDB::bind_method(D_METHOD("post"), &_Semaphore::post);
 }
 
-_Semaphore::_Semaphore() {
-
-	semaphore = SemaphoreOld::create();
-}
-
-_Semaphore::~_Semaphore() {
-
-	memdelete(semaphore);
-}
-
 ///////////////
 
 void _Mutex::lock() {