Always swap in generated move operator

[dsnopek]

Currently, this is the code we generate for Dictionary's move operator:

Dictionary &Dictionary::operator=(Dictionary &&p_other) {
	::godot::internal::_call_builtin_constructor(_method_bindings.constructor_1, &opaque, &p_other);
	return *this;
}

It's simply overwriting the opaque with data from a new Dictionary, without cleaning up the data from the old Dictionary (whereas the usual idiom is to swap the internal data with p_other, which will get cleaned up when p_other is destructed).

It looks like this comes from #656 - there was an issue where destructing Dictionary's could lead to error spam, because of an ERR_FAIL_NULL() if a Dictionary is destructed with null data. (This code is still present in Dictionary::unref().) This null could get set in the move constructor of Dictionary because it was swapping opaque value with a zero'd out one, so a fix was added to just copy rather than the usual swap.

However, it's only in the move constructor where a Dictionary could end up with null data, but the fix made the same change to move operator!

Anyway, this PR removes the fix from the move operator, while leaving it in place for the move constructor

I haven't tested yet, but I think this fixes #2048

[dsnopek]

It took a little work to get the MRP from #2048 working, but I've been able to reproduce the issue, and this PR does seem to fix it for me!