Remove the "arc export" field from Differential revisions by default

Summary: This is kind of confusing (you need to specify an export format) and
not very useful now that "arc patch" has gotten pretty good. I'm leaving the
field itself in case installs want to add it back or otherwise depend on it.

Test Plan: Looked at a revision, wasn't told to export it.

Reviewers: nh, btrahan, jungejason

Reviewed By: nh

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1507