ERP5Type: don't publish workflow methods

Wrapping a method in a workflow method should not change the
publishable state the method.
If the original method is not publishable, wrapping it in a workflow
method should not make it publishable.  If the original method is
publishable, then the wrapped method should still be publishable.

This was always intended to work like this, as we can see in the code
comment in `WorkflowMethod.__init__` but was not properly tested and got
broken at some point.

It's important to restore the behavior, because workflow methods such as
`validate` should not be published, users must only be able to use the
user interface transitions freely, workflow methods transitions are
only available if developer expose them in a script - and perform the
necessary consistency and security checks in that script.