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/os/semaphore.cpp

@@ -29,17 +29,3 @@
 /*************************************************************************/
 
 #include "semaphore.h"
-
-#include "core/error_macros.h"
-
-SemaphoreOld *(*SemaphoreOld::create_func)() = 0;
-
-SemaphoreOld *SemaphoreOld::create() {
-
-	ERR_FAIL_COND_V(!create_func, 0);
-
-	return create_func();
-}
-
-SemaphoreOld::~SemaphoreOld() {
-}