access.args: remove DirectAccess._arg_error

this was redundant with the same implementation in the superclass

software/glasgow/access/direct/arguments.py

@@ -1,5 +1,4 @@
 import functools
-import argparse
 import re
 
 from .. import AccessArguments
@@ -8,10 +7,6 @@
 class DirectArguments(AccessArguments):
     # First, define some state-less methods that just add arguments to an argparse instance.
 
-    def _arg_error(self, message, *args, **kwargs):
-        raise argparse.ArgumentTypeError(("applet {!r}: " + message)
-                                         .format(self._applet_name, *args, **kwargs))
-
     def _port_spec(self, arg):
         if not re.match(r"^[A-Z]+$", arg):
             self._arg_error("{} is not a valid port specification", arg)