mds: remove mds_log_unsafe mode

mds: remove mds_log_unsafe mode

The mds_log_unsafe mode would wait for ack for some journal writes, and
safe for others.  Now that we can reply to client requests without waiting
for the journal to flush (as of ~2 years ago), this distinction is no
longer useful.  It is also more error-prone, as it complicates the code
and vastly expands the possible combinations of MDS failures and replay
scenarios we need to verify.

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