[tests] Remove rpc_zmq.py

Summary:

rpc_zmq.py is racy and fails intermittently. Remove that test file and
move the getzmqnotifications RPC test into interface_zmq.py

Backport of core PR14419.

Before this diff, running interface_zmq and rpc_zmq at the same time
would fail, because they use the same address and port.

Test Plan:

./test/functional/test_runner.py interface_zmq

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5600