Make git-annex forget --drop-dead also remove log files for keys that have been marked as dead.

This would have the benefit of reducing the size of the git-annex branch. Also it would eliminate mention of the key in the git-annex branch's history.

Although whether it would actually delete all mention of a key is a bit hard to guarantee. Can remove all the log files that are specific to a key, like presence and metadata and chunks and remote state. But, the export.log refers to trees, and somewhere in a tree there could be a reference to a dead key.

Eg, if a file was exported to a remote, and then its key marked dead, the file could still be on the remote, and the export.log needs to keep its reference to that tree, until some other tree is exported that deletes that file. So this is a not very likely, but possible, way for the git-annex branch to still mention a dead key after --drop-dead. Could rewrite the tree as well, but now it's getting complicated indeed.