stop update-plist from spewing out $OpenBSD$

[Marc Espie]

There's more logic involved in CVSTag, but for now, just prevent the
writer from outputting anything, I can kill the remaining logic later.

Index: TrackFile.pm
===================================================================
RCS file: /cvs/ports/infrastructure/lib/OpenBSD/TrackFile.pm,v
retrieving revision 1.3
diff -u -p -r1.3 TrackFile.pm
--- TrackFile.pm        13 Feb 2022 17:42:28 -0000      1.3
+++ TrackFile.pm        28 Feb 2022 18:09:03 -0000
@@ -359,5 +359,10 @@ sub prepare_backsubst
        }
 }
 
+package OpenBSD::PackingElement::CVSTag;
+sub prepare_restate
+{
+       undef;
+}
 
 1;