Remove "Contains Words" constraint from Maniphest

Summary:
Ref T12819. Obsoleted by the Ferret engine, which is unprototyping shortly.

This breaks compatibility in two ways:

• maniphest.query no longer supports "fullText" (now throws an explicit exception).
• Existing saved searches with a "Contains Words" constraint will no longer have that constraint.

It seems unlikely (?) that either of these are seeing too much use, and they should be easy to fix. I'll note them in the changelog.

Test Plan: Viewed Maniphest, no more "Contains Words" field. Called maniphest.query with "fullText", got explicit exception.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12819

Differential Revision: https://secure.phabricator.com/D18586