block: Drop the sheepdog block driver

It was deprecated in commit e1c4269763, v5.2.0.  See that commit
message for rationale.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210501075747.3293186-1-armbru@redhat.com>
ACKed-by: Peter Krempa <pkrempa@redhat.com>
17 files changed