Remove pulling wrk from travis script (#3617)

.travis.yml

@@ -109,7 +109,6 @@ before_script:
   # to determine if the suite should be installed and the current $TESTDIR test should run.
   - export RUN_TESTS=`./toolset/travis/travis_diff.py | tee /dev/tty | grep -oP "travis-run-tests \K(.*)"`
 
-  - if [ "$RUN_TESTS" ]; then docker pull techempower/tfb.wrk; fi
   - if [ "$RUN_TESTS" ]; then docker pull techempower/tfb; fi
 
   # Stop services that would claim ports we may need