don't clear diffexp on subset

client/src/reducers/differential.js

@@ -47,14 +47,6 @@ const Differential = (
         celllist1: null,
         celllist2: null,
       };
-    case "reset subset":
-    case "subset to selection":
-      return {
-        ...state,
-        diffExp: null,
-        celllist1: null,
-        celllist2: null,
-      };
     default:
       return state;
   }