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

See merge request !1005