Fix #57, add a body to (defmethod acceptor-remove-session ((acceptor acceptor) (session t)) ...)

Since the `(defmethod acceptor-remove-session ((acceptor acceptor) (session t)) ...)` docstring wasn't a docstring, being the last form of the method. I added a `(values)` as method body.

Obviously, this shouldn't change the method intended behavior and should fix both the docstring and return value problem.