DOC: Remove "Legacy functionality" page in favor of release notes

Related to #1488, !2082 (merged)

This MR removes the "legacy functionality" page (added in !2082 (merged)) and integrates all the removal information into release notes.

One of the advantages of the "legacy functionality" page was the information on the removal reasoning, but we can now easily find it from the links to the corresponding merge requests in release notes. So, I think we can simply remove the "legacy functionality" page.