Remove BACKGROUND_COLOR from ol.Map

Let the map be transparent, the background color can be set with CSS.

src/ol/framestate.js

@@ -7,7 +7,6 @@ goog.provide('ol.PreRenderFunction');
 
 goog.require('goog.vec.Mat4');
 goog.require('ol.Attribution');
-goog.require('ol.Color');
 goog.require('ol.Extent');
 goog.require('ol.Size');
 goog.require('ol.TileQueue');
@@ -20,7 +19,6 @@ goog.require('ol.layer.LayerState');
 /**
  * @typedef {{animate: boolean,
  *            attributions: Object.<string, ol.Attribution>,
- *            backgroundColor: ol.Color,
  *            coordinateToPixelMatrix: goog.vec.Mat4.Number,
  *            extent: (null|ol.Extent),
  *            focus: ol.Coordinate,