tools/offcputime.py: Redundatn arg check
tools/offcputime.py: Redundatn arg check

pid/tid is already set to be mutually exclusive in
python's arg parser so no need for this check manually.

brendangregg/bcc,iovisor/bcc,iovisor/bcc,tuxology/bcc,tuxology/bcc,tuxology/bcc,brendangregg/bcc,tuxology/bcc,brendangregg/bcc,iovisor/bcc,iovisor/bcc,brendangregg/bcc,brendangregg/bcc,iovisor/bcc,tuxology/bcc