Core: Always enable ptrcall, remove PTRCALL_ENABLED define

ptrcall is now also used to optimize calls in GDScript, on top of the existing
use by the GDNative and Mono modules.

It no longer makes sense to make it optional.

core/object/reference.h

@@ -252,8 +252,6 @@ public:
 	WeakRef() {}
 };
 
-#ifdef PTRCALL_ENABLED
-
 template <class T>
 struct PtrToArg<Ref<T>> {
 	_FORCE_INLINE_ static Ref<T> convert(const void *p_ptr) {
@@ -272,8 +270,6 @@ struct PtrToArg<const Ref<T> &> {
 	}
 };
 
-#endif // PTRCALL_ENABLED
-
 #ifdef DEBUG_METHODS_ENABLED
 
 template <class T>