ReplicatedPG::mark_all_unfound_lost: delete local copy if necessary

There might be a local copy for an EC pool in the DELETE case.  The replica
copies should be already handled by merge_log.

Fixes: #7439
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge remote-tracking branch 'origin/wip-7439' into wip-sam-testing