If QoS service plugin is not loaded, the L3 QoS extension in the OVN
L3 plugin should not be loaded neither.
Prior to this patch, the extension drivers were checked to find the
QoS extension. Although it is a misconfiguration to have the QoS
extension without loading the QoS driver, that is handled correctly
by the Neutron server, disabling the extension silently.
Closes-Bug: #1898842
Related-Bug: #1877408
Change-Id: Iea5ff76fe652ab1c04e23850b9259547c1d54365