Remove deprecated revocation notification via zmq

[ansasaki]

Remove the deprecated notification via zmq.

The goal is to remove deprecated functionality before the next major release.

[arkivm]

@ansasaki should zeromq support be removed from https://github.com/keylime/keylime/blob/4af9457575d60b22738e5b788cfd850ed92503f6/keylime/revocation_notifier.py too?

[THS-on]

@arkivm yes, but the agent side first, then the Python side.