makeman: don't insert a $FreeBSD$

There's no need to insert this line which won't be expanded in git.  Not
generating it won't impact MFCs to 12 since the removal won't be MFCd.

Reviewed by:	emaste
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D38989

tools/build/options/makeman

@@ -140,11 +140,9 @@ main()
 {
 	echo "building src.conf.5 man page from files in ${PWD}" >&2
 
-	fbsdid='$'FreeBSD'$'
 	generated='@'generated
 	cat <<EOF
 .\" DO NOT EDIT-- this file is $generated by tools/build/options/makeman.
-.\" ${fbsdid}
 .Dd $(echo $(LC_TIME=C date +'%B %e, %Y'))
 .Dt SRC.CONF 5
 .Os