Don't make an expensive, unused call to test if a viewer can reassign a task

Summary: Depends on D19224. Ref T13106. Computing this is expensive and the value is not used. This came from D15432, but we never actually shipped that feature.

Test Plan: Saw local query cost drop from 139 to 110 with no change in functionality. Grepped for removed symbols.

Maniphest Tasks: T13106

Differential Revision: https://secure.phabricator.com/D19225