Remove "Show All Context" button from Diffusion

Summary:
Ref T12616. Diffusion, only, has a "Show All Context" button which expands the full context on all changes.

I don't remember the exact history on this, but it hasn't existed in Differential for some time and no one has complained. I suspect that the "View Options > Show All Context" on each file may replace it. I can't really come up with good reasons to use it, offhand. If we want to restore it, I think global options after T1591 is promising.

{F4945561}

Test Plan:

• Loaded a commit in Diffusion, no longer saw a button.
• Grepped for relevant sigils.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12616

Differential Revision: https://secure.phabricator.com/D17843