Remove "unstable" status and T2784-specific warning message

Summary: Ref T2784. These are lookin' pretty stable. Subclasses like DiffusionGetLintMessagesConduitAPIMethod have their warnings about unstable methods, so just remove this warning in the base class.

Test Plan: Loaded /conduit, observed lack of unstable warnings. Only unstable methods are now diffusion.getlintmessages, diffusion.looksoon, and diffusion.updatecoverage.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T2784

Differential Revision: https://secure.phabricator.com/D20651