Remove the `getConfig` and `setConfig` method.

Summary: It seems that there is a lot of overlap between `getConfig` / `setConfig` and `getLinterConfigurationOptions` / `setLinterConfigurationValue` respectively.

Test Plan: `arc lint` and `arc unit`.

Reviewers: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9067