Comment out grid cols var until we get mixins back in v3.1

customize.html

@@ -492,11 +492,14 @@ base_url: "../"
     <h3>Grid system</h3>
     <p>Define your custom responsive grid.</p>
     <div class="row">
+<!--
+  Temporarily removed until mixins are restored in v3.1
       <div class="col-xs-6 col-sm-4">
         <label>@grid-columns</label>
         <input type="text" class="form-control" placeholder="12" data-var="@grid-columns">
         <p class="help-block">Number of columns in the grid.</p>
       </div>
+ -->
       <div class="col-xs-6 col-sm-4">
         <label>@grid-gutter-width</label>
         <input type="text" class="form-control" placeholder="30px" data-var="@grid-gutter-width">