Simplify "Test Plan" custom field

Summary: Ref T11114. This leaves mail integration and UI integration, but strips all the editing (now handled by EditEngine) and commit message stuff (now handled by CommitMessageField).

Test Plan: Viewed and edited test plans and test plan transactions.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17084