Remove pointless requireCapabilities() method from PhabricatorRepositoryEditor

Summary: Depends on D19582. Ref T13164. It's not possible to reach the editor without passing through a CAN_EDIT check, and it shouldn't be necessarily to manually specify that edits require CAN_EDIT by default.

Test Plan: Grepped for RepositoryEditor, verified that all callsites pass through a CAN_EDIT check.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13164

Differential Revision: https://secure.phabricator.com/D19583