test: Remove dead code from interface_zmq test

The loop removed here appears to be effectively dead code: In case get_raw_seq is behind zmq_mem_seq the loop runs and tries to get a more recent (higher) number for get_raw_seq. However, the exact number of get_raw_seq is asserted in the line above: assert_equal(get_raw_seq, 6). If the loop would actually achieve its purpose this assert would need to be racy. This does not seem to be the case and 6 appears to be the final number. zmq_mem_seq however does take some time to catch up (if it were continue to be updated). But this is not handled by the loop and does not seem to be relevant at this point in the test. The backlog is consumed a bit later in another loop that handles this correctly already.