Remove unused milestone deprecation partial

What does this MR do?

Part of: #228846 (closed)

Remove an unused partial so that we can remove the strings containing HTML from the todolist. It looks like this partial stopped being used in 8a29bcbd but it got left in the cleanup.

This is a follow up to https://gitlab.com/gitlab-org/gitlab/-/issues/217933 where we disallowed HTML in translations and moved all existing linter errors to a todolist.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done