remove unused call to $.HeadRepo in view_title template (#32317)

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`](https://github.com/go-gitea/gitea/blob/9206fbb55fd28f21720072fce6a36cc22277934c/routers/web/repo/pull.go#L1246)
- a JSON post handler that doesn't render templates
*
[`CompareDiff`](https://github.com/go-gitea/gitea/blob/9206fbb55fd28f21720072fce6a36cc22277934c/routers/web/repo/compare.go#L706)
- which can render `diff/box.tmpl` and `diff/compare.tmpl`