UX: remove alias from chat direct message channel titles (#28958)

Makes usernames in Chat Direct Message channels consistent with other areas of the app by removing the alias.

plugins/chat/spec/system/sidebar_navigation_menu_spec.rb

@@ -182,7 +182,7 @@ RSpec.describe "Sidebar navigation menu", type: :system do
         visit("/")
 
         expect(sidebar_page.dms_section.find(".channel-#{dm_channel_1.id}")["title"]).to eq(
-          "Chat with @<script>alert('hello')</script>",
+          "Chat with <script>alert('hello')</script>",
         )
       end
     end