Remove testRunner.display from windowless plugin test

This is a straggler test that https://codereview.chromium.org/224093012/
dropped in the last patch set.

BUG=345027

Review URL: https://codereview.chromium.org/231673003

git-svn-id: svn://svn.chromium.org/blink/trunk@171179 bbb929c8-8fbe-4397-9dbb-9b2b20218538