Cleanup: make Group Unlink operator use BKE_libblock_delete(), since that's what it is doing.

Previous code (same as what `BKE_libblock_free_us()` is doing when
usercount reach 0) was probably OK in that specific case, but still not
good idea, and potentially risky.

source/blender/editors/object/object_group.c

@@ -528,8 +528,7 @@ static int group_unlink_exec(bContext *C, wmOperator *UNUSED(op))
 	if (!group)
 		return OPERATOR_CANCELLED;
 
-	BKE_libblock_unlink(bmain, group, false, false);
-	BKE_libblock_free(bmain, group);
+	BKE_libblock_delete(bmain, group);
 
 	WM_event_add_notifier(C, NC_OBJECT | ND_DRAW, NULL);