Remove unused `close_connection` in Action Cable tests (#30195)

actioncable/test/connection/multiple_identifiers_test.rb

@@ -36,8 +36,4 @@ def open_connection(server:)
       @connection.process
       @connection.send :handle_open
     end
-
-    def close_connection
-      @connection.send :handle_close
-    end
 end

actioncable/test/connection/string_identifier_test.rb

@@ -38,8 +38,4 @@ def open_connection
       @connection.process
       @connection.send :on_open
     end
-
-    def close_connection
-      @connection.send :on_close
-    end
 end