### Please describe the problem.
At some point, I migrated all my WORM-backed objects to SHA1E. I then squashed my master branch to get rid of any WORM references and dropped unused objects. Recently, I noticed that the git-annex branch still has all tracking information on the old WORM keys. I tried running git-annex forget, but the old keys are not purged, even though no (local or remote) branch refers to them and no git-annex repository has the data for these keys anymore. Should such keys be purged by git-annex forget, too?

### What steps will reproduce the problem?

[[!format sh """
# If you can, paste a complete transcript of the problem occurring here.
# If the problem is with the git-annex assistant, paste in .git/annex/daemon.log
git init /tmp/repo
cd /tmp/repo
git annex init
echo hi > file
git annex add --backend=WORM file
git commit -m init
git annex migrate
git commit -m migrated
git checkout --orphan tmp
git commit -m squashed
git branch -m master -f
git annex unused
git annex dropunused 1 --force
git annex forget --drop-dead --force
git ls-tree -r git-annex | grep WORM

# End of transcript or log.
"""]]

### What version of git-annex are you using? On what operating system?
git-annex 5.20150508
Linux 4.0.4

