Remove header gradients for flat colors

Summary: Removes header gradient images for flat, CSS controlled colors. I didn't convert the "pony" colors over, going with few options for easier theme-ability.

Test Plan:
Test each color choice.

{F1063828}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15052

scripts/celerity/generate_sprites.php

@@ -29,7 +29,6 @@ $generator = new CeleritySpriteGenerator();
 $sheets = array(
   'menu' => $generator->buildMenuSheet(),
   'tokens' => $generator->buildTokenSheet(),
-  'main-header' => $generator->buildMainHeaderSheet(),
   'login' => $generator->buildLoginSheet(),
   'projects' => $generator->buildProjectsSheet(),
 );