Remove Node.isSupported

Intent to Deprecate and Remove:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/MUEjC37jFNg/hf1AHFl-4jsJ

There were many old W3C tests for isSupported, which were all removed.
These indirectly tested DOMImplementation.hasFeature, but there are many
direct tests for that which should suffice.

BUG=334970

Review URL: https://codereview.chromium.org/133703014

git-svn-id: svn://svn.chromium.org/blink/trunk@165237 bbb929c8-8fbe-4397-9dbb-9b2b20218538