mon/PGMap: remove skewed utilizatoin warning

mon/PGMap: remove skewed utilizatoin warning

This has a few problems:

1- It does not do it's analysis over CRUSH rule roots/classes, which
means that an innocent user of classes will see skewed usage (bc hdds are
more full than ssds, say)

2- It does not take degraded clusters into account, which means the warning
will appear when a fresh OSD is added.

See http://tracker.ceph.com/issues/20730

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