mds: skip redundant flush before journal segment trim

mds: skip redundant flush before journal segment trim

Back in olden times when we would would wait for acks for some journal
writes, we did an extra wait_for_safe() before discarding a journal segment
to make sure anything being discarded was safely committed in newers
segments.  These days mds_log_unsafe is always false (and
journaler_safe is true), so we can skip this check.

Signed-off-by: Sage Weil <sage.weil@dreamhost.com>