remove unused call to $.HeadRepo in view_title template (#32317)
This is only populated in
[`ParseCompareInfo`](https://github.com/search?q=repo%3Ago-gitea%2Fgitea%20%20.Data%5B%22HeadRepo%22%5D&type=code)
which is called in two handlers:
*
[`CompareAndPullRequestPost`](9206fbb55f/routers/web/repo/pull.go (L1246))
- a JSON post handler that doesn't render templates
*
[`CompareDiff`](9206fbb55f/routers/web/repo/compare.go (L706))
- which can render `diff/box.tmpl` and `diff/compare.tmpl`
