Removed tests/dexterity_discussion.rst

This was moved here from `plone.app.dexterity`, but it has the exact same contents as `tests/functional_test_behavior_discussion.rst`.
I compared again with the original in `plone.app.dexterity`, and it is indeed the same, except that the behavior name is fixed to `plone.allowdiscussion` instead of the former `plone.app.dexterity.behaviors.discussion.IAllowDiscussion`.