[functional] Remove duplicate code from parent test class
[functional] Remove duplicate code from parent test class

The server start functional test had code identical to parent class
TestPluginWorker that can be dropped. It also broke in networking-sfc
after neutron https://review.opendev.org/c/openstack/neutron/+/793899
was merged

Change-Id: I2e31be14f7b768b563d1c6f4babb08ec6c7ca9a2
Closes-Bug: #1934900

openstack/networking-sfc,openstack/networking-sfc