Remove `private` call

Below this `private` call we only have a class definition, which as we know it
is not affected (`private` only affects methods).

We discussed with @dhh the original intention.

`ActionCable::RemoteConnections::RemoteConnection` is public interface, it is
documented in the API and it is the class of objects returned by the `where`
method. Its documentation explains which is its purpose and restricted public
interface, so we keep it public and only remove the `private` call.

actioncable/lib/action_cable/remote_connections.rb

@@ -39,7 +39,6 @@ module ActionCable
       RemoteConnection.new(server, identifier)
     end
 
-    private
     # # Action Cable Remote Connection
     #
     # Represents a single remote connection found via