test: remove necessity to call create_callback_map

Remove necessity to call create_callback_map (as well as the function
itself) from the Python P2P test framework. Invoke the appropriate
methods directly.

- Easy to forget to call it and wonder why it doesn't work
- Simplifies the code
- This makes it easier to handle new messages in subclasses

qa/rpc-tests/sendheaders.py

@@ -70,7 +70,6 @@ f. Announce 1 more header that builds on that fork.
 class BaseNode(NodeConnCB):
     def __init__(self):
         NodeConnCB.__init__(self)
-        self.create_callback_map()
         self.connection = None
         self.last_inv = None
         self.last_headers = None