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>
(cherry picked from commit 7fa1e3c37f8c7fb709ae9e070a3154a7084c0584)

Conflicts:
	PendingReleaseNotes
	qa/tasks/cephfs/test_misc.py
	src/mds/MDSDaemon.cc