PG: remove old log when we upgrade log version

Otherwise the log_oid will be non-empty and the next
boot will cause us to try to upgrade again.

Fixes: #6057
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 1f851cb2489a95526de932ec6734ebf413e43490)