Block drivers should ignore BDRV_O_CACHE_WB in .bdrv_open flags,
and in the bs->open_flags.

This patch removes the code, leaving the behaviour behind as if
BDRV_O_CACHE_WB was set.
Signed-off-by: NJeff Cody <jcody@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>