mon: remove mdsmap-derived perf counters

mon: remove mdsmap-derived perf counters

Stats collectors that want this kind of information should
be looking at the cluster maps to get it.

Reasoning:
 - It's confusing to have two places to get at this sort of status
 - In the CephFS case, these counters were not that useful as they
   globbed all filesystems together.
 - It's awkward to have these stats output as perf counters of
   ceph-mon daemons, where a naive TSDB collector will not understand
   that they are really cluster-wide things.

Inspired by: http://tracker.ceph.com/issues/21311

Signed-off-by: John Spray <john.spray@redhat.com>