staging/lustre/mgc: remove server specific handlers

Remove the server specific set_info handlers (KEY_REGISTER_TARGET,
KEY_SET_FS, KEY_CLEAR_FS) and supporting functions.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>