Remove deprecated linter configuration

Summary:
Removes the deprecated method of configuring linters (via the .arcconfig file). There is one main caveat here:

• There is currently no convenient method by which to change the path for an external linter (T5057). This means that there is no direct replacement for the deprecated lint.ruby.prefix configuration. The workaround is to symlink these binaries into arcanist/externals/bin.

Test Plan: Wait a sufficient amount of time before landing this.

Reviewers: chad, epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: aik099, epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10005