Remove gradient on dropdown menus

Summary: Fixes T4732

Test Plan: Viewed a dropdown on a diff, now uses standard {$blue}

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin, chad

Maniphest Tasks: T4732

Differential Revision: https://secure.phabricator.com/D8698