Remove unused parameter.

actionpack/lib/action_dispatch/testing/integration.rb

@@ -347,7 +347,7 @@ def reset!
       # By default, a single session is automatically created for you, but you
       # can use this method to open multiple sessions that ought to be tested
       # simultaneously.
-      def open_session(app = nil)
+      def open_session
         dup.tap do |session|
           yield session if block_given?
         end