block: Remove monitor command block_passwd

Command block_passwd always fails since

Commit c01c214b69 "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 ad1324e044 "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>

qapi/block-core.json

@@ -1207,20 +1207,6 @@
 ##
 { 'command': 'query-block-jobs', 'returns': ['BlockJobInfo'] }
 
-##
-# @block_passwd:
-#
-# This command sets the password of a block device that has not been open
-# with a password and requires one.
-#
-# This command is now obsolete and will always return an error since 2.10
-#
-##
-{ 'command': 'block_passwd',
-  'data': { '*device': 'str',
-            '*node-name': 'str',
-            'password': 'str' } }
-
 ##
 # @block_resize:
 #

qapi/pragma.json

@@ -6,7 +6,6 @@
     # Commands allowed to return a non-dictionary:
     'command-name-exceptions': [
         'add_client',
-        'block_passwd',
         'block_resize',
         'block_set_io_throttle',
         'client_migrate_info',