Remove ref-names field from git_archival.txt

It turns out the way we populated the `ref-names` field causes its value to change depending on whether the commit a Git archive was built from was the head of a branch when it was built (which is often true when we first cut a release, but ceases to be true soon afterward. If someone downloads an archive *later* and tries to compare its signature with the one we released, it won’t match because this field has changed, and that’s a significant issue.

Unfortunately, there’s no good alternative that resolves this issue, so the best solution is to just remove the field. On the up-side, `describe-name` carries the more critical info about the the current tag or release version, so this isn’t a huge loss.

Thanks to @webknjaz for pointing this out: #144 (comment)

.git_archival.txt

@@ -1,4 +1,3 @@
 node: $Format:%H$
 node-date: $Format:%cI$
 describe-name: $Format:%(describe:tags=true,match=*[0-9]*)$
-ref-names: $Format:%D$