ctladm: don't require the use of "-p" with "port -r"

When removing a port, the ioctl frontend requires the "-p" argument.
But other frontends, like cfiscsi, do not. So don't require that
argument in the ctladm command. The frontend driver will report an
error if any required argument is missing.

MFC after: 2 weeks
Sponsored by: Axcient
Reviewed by: mav
Pull Request: https://github.com/freebsd/freebsd-src/pull/1279