Problem/Motivation
We currently have a slightly-awkward CSS aggregation strategy. And, for some bizarre reason, we are putting all of the System module's CSS into a separate aggregate file. That's nonsensical.
Proposed resolution
Yes, we need to load the System CSS before other module's CSS, but that can be accomplished with a 'weight' inside the existing CSS_AGGREGATE_DEFAULT file instead of a separate CSS file.
This is the third step in a proposed solution outlined at #1921610: [Meta] Architect our CSS.
Remaining tasks
This patch is blocked by #1924368: Rename old CSS constants to CSS_AGGREGATE_* and add new CSS weight constants and #1924430: Add drupal.base library for base CSS styles
After this issue's completion, we still have all the other steps outlined at #1921610: [Meta] Architect our CSS.
User interface changes
none
API changes
Removes the CSS_AGGREGATE_SYSTEM constant. Because it's dumb.