Remove obsolete page for self-hosted runner

The self-hosted runner has recently been decommissioned and removed from
our infrastructure, so its documentation is no longer needed.

Author: jdno

src/SUMMARY.md

@@ -87,7 +87,6 @@
         - [Bors](./infra/docs/bors.md)
         - [CDN](./infra/docs/cdn.md)
         - [Crater agents](./infra/docs/crater-agents.md)
-        - [Custom GitHub Actions runners](./infra/docs/gha-self-hosted.md)
         - [Dev Desktops](./infra/docs/dev-desktop.md)
         - [GitHub App for dev-desktops](./infra/docs/dev-desktop-github-app.md)
         - [Discord moderation bot](./infra/docs/discord-mods-bot.md)