remove redundant const directives

remove redundant const directives

In 1463824d8e7f72c31f1d803d7cfe2b608ccafc5c, I added some
missing "const" directives, as well as some new, redundant ones.
This removes the redundant ones.  Pointed out by Eric Blake.
* base64.c, cat.c, chcon.c, chgrp.c, chmod.c, chown.c, comm.c:
* cp.c, csplit.c, cut.c, date.c, dd.c, df.c, dircolors.c, du.c:
* env.c, expand.c, fmt.c, fold.c, groups.c, head.c, id.c:
* install.c, join.c, kill.c, ln.c, ls.c, md5sum.c, mkdir.c:
* mkfifo.c, mknod.c, mktemp.c, mv.c, nice.c, nl.c, od.c:
* paste.c, pathchk.c, pinky.c, pr.c, ptx.c, readlink.c, rm.c:
* rmdir.c, runcon.c, seq.c, shred.c, shuf.c, sort.c, split.c:
* stat.c, stty.c, su.c, sum.c, tac.c, tail.c, tee.c, timeout.c:
* touch.c, tr.c, truncate.c, tty.c, uname.c, unexpand.c, uniq.c:
* wc.c, who.c: Remove redundant const directives.
* maint.mk (sc_const_long_option): Don't require redundant "const".