Remove unused property "slug" from Workboard View controller

Summary:
Ref T4900. The Workboard view controller currently has a lot of different responsibilities (it's ~1,500 lines long) because it has to manage the board filter/sort state.

I'd like to split it up and make it easier to move some workboard features (like "move all tasks in column...") to other Controllers, so we can have smaller controllers implementing specific workflows.

I think the state handling isn't really all that bad, it just needs to be separated a little better than it currently is.

To start with, remove the unused "slug" property.

Test Plan: Searched for "slug", got no hits. This class is final and the property is private, so this is certainly unused.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T4900

Differential Revision: https://secure.phabricator.com/D20626