internal/rosa: remove unused helpers

These are no longer needed after migration.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/git.go

@@ -36,8 +36,3 @@ git \
 rm -rf /work/.git
 `, resolvconf())
 }
-
-// newTagRemote is a helper around NewViaGit for a tag on a git remote.
-func (t Toolchain) newTagRemote(url, tag, checksum string) pkg.Artifact {
-	return t.NewViaGit(url, "refs/tags/"+tag, mustDecode(checksum))
-}