Remove `repository_tree_gitaly_pagination` feature flag

What does this MR do and why?

Contributes to #340419 (closed)

Enables support for keyset pagination for api/v4/projects/:id/repository/tree endpoint.

How to test

1. Visit http://gdk.test:3000/api/v4/projects/3/repository/tree?pagination=keyset
2. In Http response header "Link" you should see a link to the next page (similar to http://gdk.test:3000/api/v4/projects/3/repository/tree?id=3&page=1&page_token=eyJmaWxlX25hbWUiOiJMSUNFTlNFIn0%3D&pagination=keyset&per_page=20&recursive=false)
3. If you follow this link, then you should see the next page.

More details: https://docs.gitlab.com/ee/api/rest/#keyset-based-pagination