block: Remove monitor command block_passwd

Command block_passwd always fails since

Commit c01c214b "block: remove all encryption handling APIs"
(v2.10.0) turned block_passwd into a stub that always fails, and
hardcoded encryption_key_missing to false in query-named-block-nodes
and query-block.

Commit ad1324e0 "block: remove 'encryption_key_missing' flag from
QAPI" just landed.  Complete the cleanup job: remove block_passwd.

Cc: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210323101951.3686029-1-armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>