Hide "Maniphest Task:" in commit template

Summary:
We prefer parsing freetext fields.
We should probably wait for {T2743}.

Test Plan:
  $ arc --conduit-uri=http://phabricator.local/ diff

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5349

src/applications/differential/field/specification/DifferentialManiphestTasksFieldSpecification.php

@@ -80,7 +80,7 @@ final class DifferentialManiphestTasksFieldSpecification
   }
 
   public function shouldAppearOnCommitMessageTemplate() {
-    return PhabricatorEnv::getEnvConfig('maniphest.enabled');
+    return false;
   }
 
   public function shouldAppearOnCommitMessage() {