minor: Remove empty pm_list.initialize().

The commit 3cfc3ca24b
made this function do nothing, but I guess that
we thought we might resurrect the click handler
in the short term.  We never did.

static/js/pm_list.js

@@ -167,5 +167,3 @@ export function update_dom_with_unread_counts(counts) {
         counts.private_message_count,
     );
 }
-
-export function initialize() {}