migration: Remove 'skipped' field from MigrationStats

The 'skipped' field of the MigrationStats struct has been deprecated
in 8.1. Time to remove it.

Deprecation commit 7b24d32634 ("migration: skipped field is really
obsolete.").

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

qapi/migration.json

@@ -23,9 +23,6 @@
 #
 # @duplicate: number of duplicate (zero) pages (since 1.2)
 #
-# @skipped: number of skipped zero pages.  Always zero, only provided
-#     for compatibility (since 1.5)
-#
 # @normal: number of normal pages (since 1.2)
 #
 # @normal-bytes: number of normal bytes sent (since 1.2)
@@ -63,16 +60,11 @@
 #     between 0 and @dirty-sync-count * @multifd-channels.  (since
 #     7.1)
 #
-# Features:
-#
-# @deprecated: Member @skipped is always zero since 1.5.3
-#
 # Since: 0.14
 ##
 { 'struct': 'MigrationStats',
   'data': {'transferred': 'int', 'remaining': 'int', 'total': 'int' ,
            'duplicate': 'int',
-           'skipped': { 'type': 'int', 'features': [ 'deprecated' ] },
            'normal': 'int',
            'normal-bytes': 'int', 'dirty-pages-rate': 'int',
            'mbps': 'number', 'dirty-sync-count': 'int',