block/vvfat: fix: drop backing

block/vvfat: fix: drop backing

Most probably this fake backing child doesn't work anyway (see notes
about it in a8a4d15c1c34d).

Still, since 25f78d9e2de528473d52 drivers are required to set
.supports_backing if they want to call bdrv_set_backing_hd, so now
vvfat just doesn't work because of this check.

Let's finally drop this fake backing file.

Fixes: 25f78d9e2de528473d52acfcf7acdfb64e3453d4
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210715124853.13335-1-vsementsov@virtuozzo.com>
Tested-by: John Arbuckle <programmingkidx@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>