Removes / unvendors some python modules:
- Remove `_aix_support.py`, which is for supporting IBM AIX OS.
- Unvendor `_pydecimal.py`.
- _pydecimal is a pure Python implementation of `decimal` module. [Importing `decimal` fallbacks](https://github.com/python/cpython/blob/main/Lib/decimal.py) to `_pydecimal` if the C-implementation `_decimal` is not available. In our case, _decimal is available, so _pydecimal will not be normally used.
- Unvendor `pydoc_data`.
- pydoc_data contains [a large (~700KB) dictionary](https://github.com/python/cpython/blob/main/Lib/pydoc_data/topics.py) for explaining python builtins. This is mostly used when `help("...")` is called.
Gyeongjae Choi