- Cleanup the unreleased directory: leave only fix-string-dequoting.md
   there, which was added after last branching (2.10).
 - Remove 2.10.0-beta1.md, 2.10.0-beta2.md, 2.8.1.md, because they are
   available at the corresponding branches.

NO_DOC=changelogs
NO_TEST=changelogs