PERF: Remove unnecessary SQL UNION query when search public channels (#19515)

It is pointless to include DM channels because the query for public
channel search restricts chatable type to `Category`.

plugins/chat/plugin.rb

@@ -421,7 +421,6 @@ after_initialize do
       following: false,
       limit: 1,
       status: :open,
-      exclude_dm_channels: true
     ).exists?
   end