QoS plugin slows down get_ports operation

[Adit Sarfaty]

When the QoS plugin is enabled, get_ports for about 10K ports takes over 10 minutes.
removing _extend_port_resource_request which was added in https://review.opendev.org/#/c/590363/
reduces the time to around 3 minutes only.
This was tested with stable/stein

The "blame" is with the code that retrieves the network object for each port (unless the port was assigned with a qos policy): https://opendev.org/openstack/neutron/src/branch/stable/stein/neutron/services/qos/qos_plugin.py#L103

This code should be improved.
Another partial fix can be to skip _extend_port_resource_request in case the driver does not support minimum-bw rules

[Adit Sarfaty (asarfaty)]

Seems like a duplication of bug 1834484