Remove friend reference to non-existing class RefBase

Commit
7ad14e7a3e6f87ddc450f7e34621eb5200808451
removed RefBase class - but still there is hanging reference in....Reference :) class (no pun intended)

core/reference.h

@@ -38,7 +38,6 @@
 class Reference : public Object {
 
 	GDCLASS(Reference, Object);
-	friend class RefBase;
 	SafeRefCount refcount;
 	SafeRefCount refcount_init;