ul(1): Remove references to colcrt(1) and nroff(1)

colcrt(1) and nroff(1) where removed in r319664.
Remove references to these commands in ul(1) man page.

PR: 244127
Reported by: freebsd@tim.thechases.com
Approved by: manpages (gbe@)
Differential Revision: https://reviews.freebsd.org/D2614