removed course folder var from mako template

common/lib/capa/capa/templates/drag_and_drop_input.html

 <section id="inputtype_${id}" class="capa_inputtype" >
     <div class="drag_and_drop_problem_div" id="drag_and_drop_div_${id}"
-     data-plain-id="${id}" data-course-folder="${course_folder}">
+     data-plain-id="${id}">
     </div>
 
     <div class="drag_and_drop_problem_json" id="drag_and_drop_json_${id}"