Remove `paginated_tree_graphql_query` feature flag

What does this MR do and why?

Contributes to #337214 (closed)

We switched from old graphql endpoint to the new one that supports pagination. This MR removes feature flag responsible for that.

Screenshots or screen recordings

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How to set up and validate locally

1. GraphQL explorer

http://localhost:3000/-/graphql-explorer

{
  project(fullPath: "root/my-awesome-project") {
    repository {
      paginatedTree {
        nodes {
          blobs {
            nodes {
              name
            }
          }
        }
      }
    }
  }
}

2. Open repository tree http://localhost:3000/root/my-awesome-project

It should load the list of files and directories without any errors

MR acceptance checklist

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