block/backup-top: drop .active

block/backup-top: drop .active

We don't need this workaround anymore: bdrv_append is already smart
enough and we can use new bdrv_drop_filter().

This commit efficiently reverts also recent 705dde27c6c53b73, which
checked .active on io path. Still it said that the problem should be
theoretical. And the logic of filter removement is changed anyway.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20210428151804.439460-25-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>