Remove unused deprecated self._config field

The storage of the parsed configs might need to be reviewed,
as the include support introduced complexity.

Due to how includes work, the order of the include is also relevant
during lookup time.

One possible solution would be to flatten the parsed config:
to compile the host/match conditions through includes during parsing.
This way the lookup could be much simpler.

paramiko/config.py

@@ -112,7 +112,6 @@ def __init__(self):
             # Or if you have arbitrary ssh_config text from some other source:
             config = SSHConfig.from_text("Host foo\\n\\tUser bar")
         """
-        self._config = []
         self._config_root = None
         self._config_by_file = {}
 
@@ -357,7 +356,7 @@ def _lookup(
                 if key not in options:
                     # Create a copy of the original value,
                     # else it will reference the original list
-                    # in self._config and update that value too
+                    # in self._config_by_file and update that value too
                     # when the extend() is being called.
                     options[key] = value[:] if value is not None else value
                 elif key == "identityfile":