Remove baselines for ref tests.

Three layout tests had PNGs checked in for them, even though they are reftests.

TBR=eseidel@chromium.org

Review URL: https://codereview.chromium.org/361803002

git-svn-id: svn://svn.chromium.org/blink/trunk@177243 bbb929c8-8fbe-4397-9dbb-9b2b20218538