process/Broker: do not reset poller_class. Fixes #967

[philfry]

With ansible 2.12.8 and up the brokers may crash on large playbooks and/or many hosts due to the use of select.select() which is limited to 1024 file descriptors. This PR removes resetting the poller_class to mitogen.core.Poller so the best poller is used instead (that should be at least select.poll() which is not limited to 1024 fds).