Revert turning off AtD for trunk.

@azaozz has already added a stub to WordPress core trunk to keep this (and other similar plugins) from crashing JS in the browser, so it should be safe to revert the short-circuit.

Reverting https://github.com/Automattic/jetpack/commit/72809cad657dc57ff18aeaa3fed1b28c07a2c8bf
See #38