Remove deprecated features for the 0.20.x branch

Summary:
Remove all the features marked as deprecated starting from 0.20.
Updated the tests accordingly.

Test Plan:

make check
./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3978