rm cartesian bg layer in range slider range plot

- it was causing pdf/eps export to fail because the <rect>
  node didn't have x/y/width/height attributes
- remove it as the range slider bg is all we need here

Author: etpinard

src/components/rangeslider/draw.js

@@ -377,7 +377,9 @@ function drawRangePlot(rangeSlider, gd, axisOpts, opts) {
 
         Cartesian.rangePlot(gd, plotinfo, filterRangePlotCalcData(calcData, id));
 
-        if(isMainPlot) plotinfo.bg.call(Color.fill, opts.bgcolor);
+        // no need for the bg layer,
+        // drawBg handles coloring the background
+        if(isMainPlot) plotinfo.bg.remove();
     });
 }