Clean up blink_skia_config.

The gyp is no longer needed, and the defines in the gypi are
no longer required.

Review URL: https://codereview.chromium.org/717363002

git-svn-id: svn://svn.chromium.org/blink/trunk@185301 bbb929c8-8fbe-4397-9dbb-9b2b20218538