remove unused fiemap code

Signed-off-by: Sage Weil <sage@inktank.com>
mon/MonCap: mds needs to subscribe to the osdmap

Usually it can get it from the OSD, but it assumes the mon will also share
and doesn't behave when it does not.

Fixes: #5767
Signed-off-by: Sage Weil <sage@inktank.com>