Revert "Mention that Ember prototype extensions are optional early."

This reverts commit b907a351e809c981de7e354002ebae0491c1e8c5.

As mentioned in
https://github.com/emberjs/website/pull/841#issuecomment-24073977 these
guides are meant for information that nearly all consumers of the
framework will need. We likely need to add a 'further reading' section
or something.