bpo-43422: Revert _decimal C API addition (GH-24960)

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.

Automerge-Triggered-By: GH:pitrou

Author: pitrou

Doc/c-api/concrete.rst

@@ -115,4 +115,3 @@ Other Objects
    coro.rst
    contextvars.rst
    datetime.rst
-   decimal.rst