remove no-longer-needed sconsbuild reference from webkitpy.

R=thakis@chromium.org
BUG=305702

Review URL: https://codereview.chromium.org/26605004

git-svn-id: svn://svn.chromium.org/blink/trunk@159276 bbb929c8-8fbe-4397-9dbb-9b2b20218538