Remove head.js (#47507)
### What?
Removes `head.js` file convention in favor of built-in SEO support through `metadata`: https://beta.nextjs.org/docs/guides/seo
### Why?
The `head.js` convention was superseded by the built-in SEO support which is designed specifically for Server Components.
### How?
Removed the `head.js` handling in app-loader and the tests for it.
fix NEXT-812 ([link](https://linear.app/vercel/issue/NEXT-812))
