maint: only push new refspecs on release.
maint: only push new refspecs on release.

* GNUmakefile (GIT_PUBLISH): Remove.
(release): Explicitly push master, release and the new release
tag from each.

Signed-off-by: Gary V. Vaughan <f9023000f29773649f3850298becb9544b5fd6a9@gnu.org>

gvvaughan/specl