R1:cd86bf01746b

Remove `metamta.differential.unified-comment-context` and explain it in ExtraConfigSetupCheck

Summary: Ref T10694. This setting no longer has any effect: we always show a limited amount of context now.

Test Plan: `grep`

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10694

Differential Revision: https://secure.phabricator.com/D15886