math/coq: drop default variable values

bsd.port.mk handles leading "V" in GH_TAGNAME:

Before the diff:
$ make -V DISTNAME
${GH_PROJECT}-${V}
$ make show=DISTNAME
coq-8.13.2

$ make -V WRKDIST
${WRKDIR}/${GH_PROJECT}-${V}
$ make show=WRKDIST
/usr/ports/pobj/coq-8.13.2/coq-8.13.2

After the diff:
$ make -V DISTNAME
${GH_PROJECT}-${GH_TAGNAME:C/^(v|V|ver|[Rr]el|[Rr]elease)[-._]?([0-9])/\2/}
$ make show=DISTNAME
coq-8.13.2

$ make -V WRKDIST
${WRKDIR}/${GH_PROJECT}-${GH_TAGNAME:C/^[vV]([0-9])/\1/}
$ make show=WRKDIST
/usr/ports/pobj/coq-8.13.2/coq-8.13.2

Feedback? Objection? OK?

Index: Makefile
===================================================================
RCS file: /cvs/ports/math/coq/Makefile,v
retrieving revision 1.57
diff -u -p -r1.57 Makefile
--- Makefile 10 Dec 2021 04:11:00 -0000 1.57
+++ Makefile 12 Feb 2022 00:32:03 -0000
@@ -6,8 +6,6 @@ V= 8.13.2
GH_ACCOUNT = coq
GH_PROJECT = coq
GH_TAGNAME = V${V}
-DISTNAME = ${GH_PROJECT}-${V}
-WRKDIST = ${WRKDIR}/${GH_PROJECT}-${V}
REVISION = 0

CATEGORIES= math