Remove actor as a follower from unowned Asana subtasks after touching them

Summary: Ref T2852. Asana adds the actor as a follower when they create a task, so subtasks currently have up to two followers (the actor and the reviewer) when they should have only one (the reviewer). Simply removing the actor is an effective remedy for this because unfollowing tasks occurs with sneaky ninja stealth in Asana and doesn't generate notifications or even transaction activity.

Test Plan: Synchronized a revision without this patch, saw two followers on the subtask. Synchronized a revision after this patch, saw the "removeFollowers" fire and only one follower on the subtask, with no record of the removal in notifications or the transaction log.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6700