remove git-crypt from a repository

Now that keybase unvailed [encrypted Git](https://keybase.io/blog/encrypted-git-for-everyone), I'm going to move my private repos from Github to Keybase. How can I decrypt the history of an entire repository? I have tried several `git filter-branch` commands without success, here's what I've tried so far:

- `git filter-branch --index-filter "git rm -rf .gitattributes .git-crypt; git update-index --really-refresh" --prune-empty -f`
- `git filter-branch --index-filter "git rm -rf .gitattributes .git-crypt" --prune-empty -f`
- `git filter-branch --index-filter "git reset HEAD && git rm -rf .gitattributes .git-crypt && git add -A ." --prune-empty -f`