CLEANUP: pattern: delete the back refs at once during pat_ref_reload()

It's pointless to delete a backref and relink it to the next entry since
the next entry is going to do the exact same and so on until all of them
are deleted. Let's simply delete backrefs on reload.
diff --git a/src/pattern.c b/src/pattern.c
index 328c514..e4a1e27 100644
--- a/src/pattern.c
+++ b/src/pattern.c
@@ -2033,15 +2033,9 @@
 	/* all expr are locked, we can safely remove all pat_ref */
 	list_for_each_entry_safe(elt, safe, &ref->head, list) {
 		list_for_each_entry_safe(bref, back, &elt->back_refs, users) {
-			/*
-			 * we have to unlink all watchers. We must not relink them if
-			 * this elt  was the last one in the list.
-			 */
-			LIST_DEL(&bref->users);
-			LIST_INIT(&bref->users);
-			if (elt->list.n != &ref->head)
-				LIST_ADDQ(&LIST_ELEM(elt->list.n, typeof(elt), list)->back_refs, &bref->users);
-			bref->ref = elt->list.n;
+			/* we have to unlink all watchers. */
+			LIST_DEL_INIT(&bref->users);
+			bref->ref = NULL;
 		}
 		LIST_DEL(&elt->list);
 		free(elt->pattern);