Remove outdated comment in ExceptionsWrapper#show? [ci skip]

Support for `true` and `false` was removed in
ec2c266.

Author: eugeneius

actionpack/lib/action_dispatch/middleware/exception_wrapper.rb

@@ -180,7 +180,6 @@ def show?(request)
       # once.
       config = request.get_header("action_dispatch.show_exceptions")
 
-      # Include true and false for backwards compatibility.
       case config
       when :none
         false