rgw multisite: obsolete 'radosgw-admin period prepare' command

rgw multisite: obsolete 'radosgw-admin period prepare' command

All that the 'radosgw-admin period prepare' command does is create an
empty period and write it to rados. When we added the staging period
(see 'period get --staging'), uncommitted changes were written to that
instead - so the 'period prepare' command became obsolete. It should be
removed!

Fixes: http://tracker.ceph.com/issues/17387
Reported-by: Casey Bodley <cbodley@redhat.com>
Signed-off-by: Gaurav Kumar Garg <garg.gaurav52@gmail.com>