Remove check for custom_exceptions from showtraceback; it's checked in run_code.

IPython/core/interactiveshell.py

@@ -1703,9 +1703,6 b' class InteractiveShell(SingletonConfigurable):'
             elif etype is UsageError:
                 self.write_err("UsageError: %s" % value)
             else:
-                if etype in self.custom_exceptions:
-                    stb = self.CustomTB(etype, value, tb, tb_offset)
-                else:
                     if exception_only:
                         stb = ['An exception has occurred, use %tb to see '
                                'the full traceback.\n']