R1:c7cd844a07f8

Remove deprecated task subscriber class

Summary: This class is no longer used after D10965.

Test Plan: `grep`

Reviewers: btrahan, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11133