Remove repository graph log to fix slow cache updates after push event

Recalculating graph_log took minutes on a large repository since each of the 6000 commits would need to calculate the diff. Since graph_log does not appear to be used, remove it from the list of things to update. For the Linux kernel repository, this reduced the ProjectCacheWorker time from 8+ minutes to 16 seconds.

Closes #2016 (closed)