lib.wiring: Component.signature should not be a class method.

While the capability of providing signatures for components that are not
parametric is useful, most Amaranth gateware is heavily parameterized,
and the capability is not worth making most subclasses Liskov-incompatible
with the base class (where the derived class would not provide `signature`
as a class method anymore).

amaranth/lib/wiring.py

@@ -782,9 +782,6 @@ class Component(Elaboratable):
                                 f"because an attribute with the same name already exists")
         self.__dict__.update(self.signature.members.create())
 
-    # TODO(py3.9): This should be a class method, but descriptors don't stack this way
-    # in Python 3.8 and below.
-    # @classmethod
     @property
     def signature(self):
         cls = type(self)