Remove WTF_USE_HARFBUZZ

Remove the WTF_USE_HARFBUZZ define and the remaining use of it as it is
it only used in files compiled on platforms where it is guaranteed to be
true.

Review URL: https://codereview.chromium.org/416273004

git-svn-id: svn://svn.chromium.org/blink/trunk@179185 bbb929c8-8fbe-4397-9dbb-9b2b20218538