[Work-Around] Remove Extra Error Class in MADXParser

Sorting issue in pybind11-stubgen can be resolved by:
- moving the base class into a separate file/import
- renaming the base class to something alphabetically earlier
- removing the base class.

Since the class that shows the issue is somewhat unnecessary, we
remove it.

src/python/impactx/MADXParser.py

@@ -14,11 +14,7 @@
 import warnings
 
 
-class MADXParserError(Exception):
-    pass
-
-
-class MADXInputError(MADXParserError):
+class MADXInputError(Exception):
     def __init__(self, args, with_traceback):
         self.args = args
         self.with_traceback = with_traceback