mon: fix 'pg dump' JSON output

This was broken by 40bdcb88.  The 'acting' array had
the up_primary and acting_primary appended.

Fixes: #7572

Signed-off-by: John Spray <john.spray@inktank.com>
Merge pull request #1339 from ceph/wip-7572

mon: fix 'pg dump' JSON output

Reviewed-by: Loic Dachary <loic@dachary.org>