Remove Schnorr activation

Summary:
This sets the SCRIPT_ENABLE_SCHNORR flag to be on for all time, retroactively.
The Schnorr activation test is simplified to a feature test.

(effectively this is a backing-out of D2483)

Test Plan:
make check
test_runner.py --extended

• IBD with -checkpoints=0 and -assumevalid=0 on testnet
• IBD with -checkpoints=0 and -assumevalid=0 on mainnet

Reviewers: Mengerian, jasonbcox, Fabien, #bitcoin_abc, deadalnix

Reviewed By: Mengerian, jasonbcox, #bitcoin_abc, deadalnix

Maniphest Tasks: T653

Differential Revision: https://reviews.bitcoinabc.org/D3253