Remove `groff' from USES.

It was recently killed in the base system and consumers have to use
`textproc/groff' port now, which is a real PITA: it pulls in some
obscure dependencies like print/psutils', graphics/netpbm', but
most importantly, `ghostscript' (unconditionally), which is a huge
abomination of its own, pulling another round of dependencies, some
of which are as crazy as `graphics/svgalib' (sic). Trying to reset
all offered knobs to `off' results in unbuildable package.

Pass NROFF as mere `echo' and be done with it. Generated manpages
remain identical.