Remove Kubo (userland) specifications

[hacdias]

A few of the specifications in this repository are not part of the "IPFS" specifications. Some are simply the specification of what certain IPFS implementations do. I'm opening this issue because I want to remove those from this repository and move them to the correct places.

These are the ones I identified:

• https://github.com/ipfs/specs/blob/main/REPO.md seems to be kubo and https://github.com/ipfs/js-ipfs-repo specific. I think we should move it to kubo, as I don't think there's anything we can call an "IPFS repository".
• https://github.com/ipfs/specs/blob/main/REPO_FS.md, same as above, but for specific implementation.
• https://github.com/ipfs/specs/blob/main/KEYSTORE.md is kubo specific

Am I missing anything? cc @lidel