staging/lustre/mgc: inline mgc_cancel()

The function mgc_cancel() is never invoked as an OBD device method and
is only called directly from mgc_process_log() so remove it.

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