Remove the "Graph" view as a dedicated repository view
Ref T13552. Currently, Diffusion has two effectively identical history views, the "Graph" view and the "History" view.
These arose out of product uncertainty about the importance of the graph, but I think we can just put the graph on the "object item list" view and merge these views.
Test Plan: Looked at repositories in Diffusion, no longer saw a "Graph" tab. Grepped for "graph"-related symbols.
Maniphest Tasks: T13552
Differential Revision: https://secure.phabricator.com/D21409