Previously, it would return an error instead. Now we can distinguish
the cases 'error while deleting known key' vs. 'key not in index'
without testing for leveldb-internal kinds of errors.

persistence.go is way too long anyway, and a lot of code is just crash
recovery, which is not important to understand the normal operation.

Also, remove unused `exists` function.

Previously, those were simply deleted. The orphaned files can now be
used for forensics if needed.