bindings: Remove 'AllowJSCreationOnlyIfFeatureEnabled' IDL attribute.

The attribute was introduced as a speculative fix for crbug.com/256230, but it is
unnecessary. So remove the attribute.

BUG=256230
R=haraken@chromium.org

Review URL: https://codereview.chromium.org/19818003

git-svn-id: svn://svn.chromium.org/blink/trunk@154768 bbb929c8-8fbe-4397-9dbb-9b2b20218538