Revert "block: Restructure remove_file_or_backing_child()"

That's a preparation to previously reverted
"block: Let replace_child_noperm free children". Drop it too, we don't
need it for a new approach.

This reverts commit 562bda8.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220726201134.924743-12-vsementsov@yandex-team.ru>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -5055,33 +5055,30 @@ static void bdrv_remove_file_or_backing_child(BlockDriverState *bs,
                                               BdrvChild *child,
                                               Transaction *tran)
 {
-    BdrvChild **childp;
     BdrvRemoveFilterOrCowChild *s;
 
+    assert(child == bs->backing || child == bs->file);
+
     if (!child) {
         return;
     }
 
-    if (child == bs->backing) {
-        childp = &bs->backing;
-    } else if (child == bs->file) {
-        childp = &bs->file;
-    } else {
-        g_assert_not_reached();
-    }
-
     if (child->bs) {
-        bdrv_replace_child_tran(*childp, NULL, tran);
+        bdrv_replace_child_tran(child, NULL, tran);
     }
 
     s = g_new(BdrvRemoveFilterOrCowChild, 1);
     *s = (BdrvRemoveFilterOrCowChild) {
         .child = child,
-        .is_backing = (childp == &bs->backing),
+        .is_backing = (child == bs->backing),
     };
     tran_add(tran, &bdrv_remove_filter_or_cow_child_drv, s);
 
-    *childp = NULL;
+    if (s->is_backing) {
+        bs->backing = NULL;
+    } else {
+        bs->file = NULL;
+    }
 }
 
 /*