Remove or enable unconditionally EXTRA_FUNCTIONALITY stuff from the _decimal module

Bug report

Bug description:

I suspect this define was added to make some staging area for new functionality. Though, it has no pure-Python alternative, not tested by CI or build bots, has regressions (latest: https://github.com/python/cpython/issues/152870).

We should either remove this or include unconditionally, with appropriate support from _pydecimal.py.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

まず、_decimal モジュールでの EXTRA_FUNCTIONALITY の処理を、_pydecimal.py における対応するサポートと比較します。regression issue 152870 と、現在の CI または build-bot のカバレッジを確認します。staging 機能を削除するか無条件で有効化し、pure-Python 実装とリグレッションが適切にカバーされれば完了です。