bpo-43422: Revert _decimal C API addition

[pitrou]

Stefan Krah requested the reversal of these (unreleased) changes, quoting him:

The capsule API does not meet my testing standards, since I've focused
on the upstream mpdecimal in the last couple of months.
Additionally, I'd like to refine the API, perhaps together with the
Arrow community.

https://bugs.python.org/issue43422

Automerge-Triggered-By: GH:pitrou

[pitrou]

These changes was already reviewed in #24792, except for the trivial addition of a NEWS item.

[miss-islington]

@pitrou: Status check is done, and it's a success ✅ .