Remove useless template file (go-gitea#29053)

Removed `templates/repo/settings/nav.tmpl`.

I don't think it's still used.

On main branch:
<img width="521" alt="image"
src="https://github.com/go-gitea/gitea/assets/9418365/c0bf958c-698a-4348-840e-3ade0312bd6e">

On commit 755eec7 (the commit that
created this file):

<img width="615" alt="image"
src="https://github.com/go-gitea/gitea/assets/9418365/5156b26d-1ecd-4197-a0a6-dc2d17652ff4">