dbcheck: don't check expired tombstone objects by default anymore

dbcheck: don't check expired tombstone objects by default anymore

These will be removed anyway and any change on them risks to
be an originating update that causes replication problems.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13816

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Thu Mar 14 03:12:27 UTC 2019 on sn-devel-144

(cherry picked from commit a2c5f8cf41c2dfdc4f122e8427d1dfeabb6ba311)