Remove the bindings element

[mattgarrish]

Consensus to remove the bindings element from EPUB 3.1 was reached on the 2016-01-08 conference call (https://docs.google.com/document/d/1Ari3a4urma6js0L-GLkzwDEqsI2dKFPs7X1J9LXqNEk/edit#heading=h.zc0soncmn95).

If foreign media types are referenced from the object element, the intrinsic fallback of markup within the object element remains. Removing bindings only removes an option for the reading system to invoke a scripted content document, but this feature is not widely used or supported.

This issue will remain open past publication of the first draft for comment.