Remove more scrolling code from Sky

None of this code does anything anymore.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/878303002

examples/flights-app/app-scrollable.sky

@@ -6,7 +6,7 @@
     :host {
       flex: 1;
       box-shadow: inset 0px 0px 22px 2px rgba(22, 22, 22, 0.63);
-      overflow: scroll;
+      overflow: hidden;
     }
   </style>
   <content></content>

examples/flights/index.sky

@@ -52,7 +52,7 @@
     app-scrollable {
       flex: 1;
       box-shadow: inset 0px 0px 22px 2px rgba(22, 22, 22, 0.63);
-      overflow: scroll;
+      overflow: hidden;
     }
 
     app-panel {