Remove unused macro ENABLE_CUSTOM_SCHEME_HANDLER

This patch removes the unused macro ENABLE_CUSTOM_SCHEME_HANDLER.

Review URL: https://codereview.chromium.org/518283002

git-svn-id: svn://svn.chromium.org/blink/trunk@181256 bbb929c8-8fbe-4397-9dbb-9b2b20218538