Fix crash caused by GDScript self unreferencing during reload

[Xrayez]

Fixes #41393, caused by 8088e9e.

Saving stuff in the editor runs through threads as well, which likely explains crashes to happen sporadically.

I'm not sure if manual unreferencing is needed for something else, but I couldn't reproduce the crash with this change anymore.

[Xrayez]

I think that self.unref() was meant to be used in this case, but then again it's not really needed to do this manually:

godot/core/reference.h

Lines 233 to 235 in 0ee0fa4

	~Ref() {
	unref();
	}

Also found some interesting remark for mutexes:

godot/core/reference.h

Lines 216 to 225 in 0ee0fa4

	void unref() {
	//TODO this should be moved to mutexes, since this engine does not really
	// do a lot of referencing on references and stuff
	// mutexes will avoid more crashes?
	if (reference && reference->unreference()) {
	memdelete(reference);
	}
	reference = nullptr;
	}

[vnen]

The problem is that the cache was holding references so it needs to decrease the counter, otherwise they will leak. I changed how the cache works in #41547 to not keep an extra reference so this is not needed anymore.

[Xrayez]

Oki, thanks for response, should be superseded by #41547.