Hide speaker note popup link on mobile

This uses the same media query as the rest of the mdbook theme:
devices with a width less than 1080px (mobiles) will not see the link
to open speaker notes in a new window.

speaker-notes.css

@@ -22,3 +22,9 @@
 .content details summary a {
   margin-left: 0.5em;
 }
+
+@media only screen and (max-width: 1080px) {
+  .content details summary a {
+    display: none;
+  }
+}