N+1 problems on milestone page

What does this MR do?

Fixes N+1 problems on a project milestone page as follows:

1. Avoid to repeat namespace querying on project milestone (cf. !10871 (merged))
2. Remove issuable drag-n-drop and sorting from milestone view completely (introduced in !7113 (merged) (#13670 (closed)), but later incompletely removed in !11950 (merged))
3. Remove some pieces of duplicated code

Are there points in the code the reviewer needs to double check?

No

Why was this MR needed?

A project milestone page is very slow. e.g., https://gitlab.com/gitlab-org/gitlab-ce/milestones/37

Screenshots (if relevant)

No

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] API support added
• Tests
  • [n/a] Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #34736 (closed)