Remove the "Graph" view as a dedicated repository view

Summary:
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