indent(1): remove troff output support

The troff output in indent was invented at Sun and the online documentation
for some post-SunOS operating system includes this:
The usual way to get a troffed listing is with the command

indent -troff program.c | troff -mindent

The indent manual page in FreeBSD 1.0 already lacks that information and
troff -mindent complains about not being able to find the macro file.
It seems that the file did exist on SunOS and was supposed to be imported
into 4.3BSD together with the feature, but that has never happened.

Removal of troff output support simplifies a lot of indent's code.

vgrind(1) seems to be a promising replacement.