doc: Remove unused documentation about unimplemented features
doc: Remove unused documentation about unimplemented features

Summary:
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/19090 | core#19090]] [4/6]
https://github.com/bitcoin/bitcoin/pull/19090/commits/fa9cca0550f3d0ee8c276146f40007f76fbb97c2

Test Plan: NA

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9958

Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc