Workday: fix LLCONFLICT when deparenting a task.

Summary:
dissociate() includes an ItemUpdateJob, so don't run a second one
in parallel.

Note that the inbox code only calls dissociate, which is why it doesn't
run into the LLCONFLICT.

Test Plan: Now dissociating works, in the workday.

Reviewers: ervin

Reviewed By: ervin

Subscribers: Zanshin

Differential Revision: https://phabricator.kde.org/D9355