R1:2ee5e71029af

Simplify implementation of "SysetemAction->getSystemActionConstant()"

Summary: Depends on D20668. Ref T13343. Just an easy cleanup/simplification while I'm here.

Test Plan: `grep` for `getActionConstant()`

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13343

Differential Revision: https://secure.phabricator.com/D20669