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.

frontend_tests/node_tests/pm_list.js

@@ -100,7 +100,6 @@ run_test("build_private_messages_list", (override) => {
     expected_data[0].is_zero = true;
     assert.deepEqual(pm_data, expected_data);
 
-    pm_list.initialize();
     pm_list._build_private_messages_list();
     assert.deepEqual(pm_data, expected_data);
 });

static/js/pm_list.js

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

static/js/ui_init.js

@@ -38,7 +38,6 @@ import * as overlays from "./overlays";
 import * as panels from "./panels";
 import * as people from "./people";
 import * as pm_conversations from "./pm_conversations";
-import * as pm_list from "./pm_list";
 import * as presence from "./presence";
 import * as recent_topics from "./recent_topics";
 import * as reload from "./reload";
@@ -526,7 +525,6 @@ export function initialize_everything() {
     unread_ui.initialize();
     activity.initialize();
     emoji_picker.initialize();
-    pm_list.initialize();
     topic_list.initialize();
     topic_zoom.initialize();
     drafts.initialize();