Mk/Uses/go.mk: Drop PLIST_SUB for GO_PKGNAME

GO_PKGNAME defaults to ${PORTNAME}. As a result it is often too
general and can produce bad results with make makeplist. For
example, in mail/aerc it produces bogus entries like

man/man5/%%GO_PKGNAME%%-sendmail.5.gz

since GO_PKGNAME=aerc there.

Having it in PLIST_SUB was only useful for Go libraries which we
no longer support. Only 2 ports use it currently but do not really
need to and are easily fixed.

Approved by: dg@syrec.org (maintainer)
Differential Revision: https://reviews.freebsd.org/D21114