refactor: Remove dead code in PM list.

The function that was called here has no side
effects.  If you don't use its value, it's just
wasted computation.  The real action happens
in the subsequent calls to `rebuild_recent`.

static/js/pm_list.js

@@ -152,8 +152,6 @@ exports.rebuild_recent = function (active_conversation) {
 };
 
 exports.update_private_messages = function () {
-    exports._build_private_messages_list();
-
     if (!narrow_state.active()) {
         return;
     }