mds: remove cache drop asok command

mds: remove cache drop asok command

`cache drop` is a long running command that will block the asok interface
(while the tell version does not). Attempting to abort the command with ^C or
equivalents will simply cause the `ceph` command to exit but won't stop the
asok command handler from waiting for the cache drop operation to complete.

Instead, just allow the tell version.

Fixes: http://tracker.ceph.com/issues/38020
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>