Simplify default field loadout for Differential

Summary:
Remove the blame revision, revert plan and lines fields from the default field
loadout. (After D829 this doesn't cause issues where we have bogus dictionary
entries.)

You should add these back to the Facebook configuration since Facebook wants
these fields. However, I want to keep the default stack very light and I never
saw a huge amount of value in these fields at Facebook so I don't think they
make the cut. Sorry, tomo. ;_;

Test Plan: Ran "arc diff" locally.

Reviewers: jungejason, tuomaspelkonen, aran

Reviewed By: aran

CC: aran, tomo, epriestley

Differential Revision: 831