Throw out readKeys for a group when we're excluded

[Powersource]

We've been doing that but in #101 (comment) we discovered that that breaks some index/db2 code, so now I'm gonna change it so we just keep readKeys for a group we're excluded from.

Since the readKeys are sent to us in public (in that they're on non-hidden feeds) messages encrypted as DMs to us, it's trivial to find the readKeys again. So this isn't super high priority. But there's no reason to keep them in storage, so we should throw the keys out.

This is a separate issue from #102. That issue is about deleting the group feeds from the group. But there are group related messages that are not on the group feeds, such as the group feed announce messages, and the member addition messages on the additions feeds. #102 should be done before this issue, since it's both simpler and has a bigger impact.

[Powersource]

The PRs where we start keeping the readKeys (and root) https://gitlab.com/ahau/lib/ssb-keyring/-/merge_requests/26 and ssbc/ssb-box2#32

I think this issue would involve basically reverting those. As well as fixing the index/db2 bug that breaks on suddenly missing readKeys.