Don't link to specific WHATWG multipage page

"Links to the multipage version of the specification are unfortunately
likely to break over time."
-- https://html.spec.whatwg.org/multipage/asefij.html

This commit removes all references to the specific pages when viewing
WHATWG using multipage mode. I went through all these links and they
redirect fine.

Regex used to generate this commit:

`s_whatwg.org/multipage/.*#_whatwg.org/multipage/#_g`

components/script/dom/webidls/Storage.webidl

@@ -4,7 +4,7 @@
  * You can obtain one at http://mozilla.org/MPL/2.0/.
  *
  * The origin of this IDL file is
- * https://html.spec.whatwg.org/multipage/webstorage.html#webstorage
+ * https://html.spec.whatwg.org/multipage/#webstorage
  *
  */
 

components/script/dom/webidls/StorageEvent.webidl

@@ -4,7 +4,7 @@
  * You can obtain one at http://mozilla.org/MPL/2.0/.
  *
  * Interface for a client side storage. See
- * https://html.spec.whatwg.org/multipage/webstorage.html#the-storageevent-interface
+ * https://html.spec.whatwg.org/multipage/#the-storageevent-interface
  * for more information.
  *
  * Event sent to a window when a storage area changes.

components/script/dom/webidls/Window.webidl

@@ -128,14 +128,14 @@ partial interface Window {
 };
 Window implements OnErrorEventHandlerForWindow;
 
-// https://html.spec.whatwg.org/multipage/webstorage.html#dom-sessionstorage
+// https://html.spec.whatwg.org/multipage/#dom-sessionstorage
 [NoInterfaceObject]
 interface WindowSessionStorage {
   readonly attribute Storage sessionStorage;
 };
 Window implements WindowSessionStorage;
 
-// https://html.spec.whatwg.org/multipage/webstorage.html#dom-localstorage
+// https://html.spec.whatwg.org/multipage/#dom-localstorage
 [NoInterfaceObject]
 interface WindowLocalStorage {
   readonly attribute Storage localStorage;