Back in Newton, patch [1] added to the agents possibility to report in
the heartbeat messages if hybrid plug of the ports is required or not.
Usage of "firewall_driver" option by mechanism drivers (so on the
server's side) was kept just for backward compatibility.
But as we are now about 4 years from the [1] I think it should be safe
to do small cleaning, remove usage of this option in the neutron server
and not confuse users where this config option has to be set and why.
[1] https://review.opendev.org/#/c/311814/
Change-Id: I2ccc4c8784c64858acaa3c3431cf9a3d13e5e154
740ab628f4