Fix task + commit associating

Summary: we weren't actually removing any edges. now we do.

Test Plan: had a commit associated with task x; removed association. had a commit associated with task x; removed association while adding a different one.

Reviewers: floatinglomas, epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Maniphest Tasks: T1256

Differential Revision: https://secure.phabricator.com/D2509