TT#70901 selenium: dont start geckodriver explicitly in testrunner script

- python selenium is perfectly capable to start geckodriver by itself, so we
can remove geckodriver launch in shell script. we are still killing geckodriver
on launch if necessary to prevent issues with re-execution

Change-Id: I4e413abfd3d4a2f04bee6d48bd33654cc559f562

t/selenium/testrunner

@@ -43,7 +43,6 @@ firefox --version
 
 # ensure we don't leave any process behind causing problems with re-execution
 pkill -f 'geckodriver' || true
-DISPLAY=:99 ~/geckodriver -p 4444 --log fatal &
 
 echo "################################################################################"
 echo "Finished main setup, now running tests ..."