Remove invokable.py because it is unused, and most likely outdated

Signed-off-by: Stefan Marr <git@stefan-marr.de>

src/som/vmobjects/invokable.py

-class Invokable(object):
-    """An 'interface', or common super class for methods and primitives.
-       WARNING: This interface is not used in the RPython version, because
-                RPython does not support multiple inheritance.
-                It is kept merely for informative purposes to show the common
-                parts of Method and Primitive.
-    """
-
-    # Tells whether this is a primitive
-    def is_primitive(self):
-        raise NotImplementedError()
-
-    # Invoke this invokable object in a given frame
-    def invoke(self, frame, interpreter):
-        raise NotImplementedError()
-
-    # Get the signature for this invokable object
-    def get_signature(self):
-        raise NotImplementedError()
-
-    # Get the holder for this invokable object
-    def get_holder(self):
-        raise NotImplementedError()
-
-    # Set the holder for this invokable object
-    def set_holder(self, value):
-        raise NotImplementedError()