Remove HAVE_WEBUNITTESTSUPPORT.

This macro is defined but never used, and appears to be a vestige of
landing https://bugs.webkit.org/show_bug.cgi?id=109369 that was never
cleaned up.

BUG=

Review URL: https://codereview.chromium.org/212733002

git-svn-id: svn://svn.chromium.org/blink/trunk@170087 bbb929c8-8fbe-4397-9dbb-9b2b20218538