Don't dispatch update jobs on deleted objects

Summary: we could use saveQueietly() in DeleteJobs, but this is a more generic approach.

Test Plan: ./phpunit

Reviewers: #hkccp_developers, mollekopf

Reviewed By: #hkccp_developers, mollekopf

Subscribers: #hkccp_developers

Differential Revision: https://git.kolab.org/D5022