delete leftover index temp files

[sourcefrog]

If a backup is interrupted, temporary files might be left behind in the index directory. These can be deleted immediately before the band is finalized.

Possibly, conserve gc could also look for them in already-complete index directories. They might have been left behind by versions before the check in the first paragraph was added.

There probably can't ever be very much space used by these, so it's not a high priority.