Set default value of showanswer to Never since it doesn't work.

TNL-645

common/lib/xmodule/xmodule/templates/problem/drag_and_drop.yaml

@@ -2,6 +2,7 @@
 metadata:
     display_name: Drag and Drop
     markdown: !!null
+    showanswer: never
 data: |
     <problem>
       <p>

common/lib/xmodule/xmodule/templates/problem/jsinput_response.yaml

@@ -2,6 +2,7 @@
 metadata:
     display_name: Custom Javascript Display and Grading
     markdown: !!null
+    showanswer: never
 data: |
       <problem>
       <p>