Stop building Route rows on the fly

What does this MR do?

This MR ensures that all existing projects and namespaces have a route, and removes the code for dynamically generating routes. This simplifies the code, and reduces the number of queries necessary in case a Project or Namespace does not yet have a route. We also no longer need to expire path caches, since we no longer cache them in the first place.

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6591

Why was this MR needed?

This is explained in https://gitlab.com/gitlab-org/gitlab-ce/issues/44390.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• Conform by the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/44390