docs: Remove Halloween promotional background for Getting Started video

- remove background image style and image file
- restore section color

Change-Id: I3c3a4b2e1e3463f46f3a3f67d0331faeb64bdce4

docs/html/develop/index.jd

@@ -9,7 +9,7 @@ tabbedList=1
 excludeFromSuggestions=true
 @jd:body
 
-<section class="dac-expand dac-hero dac-section-light dac-invert" style="background-image: url(/images/tools/walking-threads-bg.jpg)">
+<section class="dac-expand dac-hero dac-section-light">
   <div class="wrap">
     <div class="cols dac-hero-content">
       <div class="col-1of2 col-push-1of2 dac-hero-figure">