makeman: don't copy $FreeBSD$ tags from source files into output

Copying the source VCS ID from WITH_/WITHOUT_* into the generated
src.conf.5 is not necessary, as it is generally possible to determine
the same information using the VCS to examine commit metadata. The
individual source files also match the name of the option recorded in
the generated file, so it is clear from where the content originated.

The copied IDs generate extraneous, non-content noise in the generated
src.conf.5 in some cases, including the first time the file is generated
on a stable branch. In addition, the source file $FreeBSD$ tags are not
expanded by git or other non-Subversion VCSs, which means that makeman
previously could not be used in a non-Subversion working tree.

I accept that there's some desire to keep these, but on balance believe
there is more benefit in removing them.

Reviewed by: imp, ngie
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D7997