Cleanup: Remove blink_test_runner.gyp

Its target was moved into public/blink.gyp:blink_web_test_support (in r166798
- https://codereview.chromium.org/158243002/) and
Chromium was update to use the new target (in r250764 -
https://codereview.chromium.org/147683010/) so we can remove this now

BUG=None
TEST=None
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/165873002

git-svn-id: svn://svn.chromium.org/blink/trunk@167153 bbb929c8-8fbe-4397-9dbb-9b2b20218538