OCM does not cover revoking invitations

[butonic]

We can currently only send invites to remote instances, but I do not see a way to notify a remote instance that an invite was revoked.

Might be out of scope, but we stumbled over this during QA.

[glpatcern]

hey @butonic, in fact that would be covered by (optional) notifications:

https://github.com/cs3org/OCM-API/blob/develop/spec.yaml#L664

Where USER_REMOVED would tell the other end about this revocation. Yet, as far as I know no EFSS implements this.