Fix asset_path in mounted engine

This is my solution for #8119, more in commit message:

```
Historically serving assets from a mountable engine could be achieved by
running ActionDispatch::Static as a part of engine middleware stack or
to copy assets prefixed with an engine name. After introduction of
assets pipeline this is not needed as all of the assets are served or
compiled into main application's assets.

This commit removes the obsolete line making asset_path always generate
paths relative to the root or config.relative_url_root if it's set.

(closes #8119)
```