remove effect of SCRIPT_ENABLE_SCHNORR flag

Summary:
Originally, the SCRIPT_ENABLE_SCHNORR flag was added first without any
effect, then later diffs put in the functionality. This Diff removes the
effect of the SCRIPT_ENABLE_SCHNORR flag, but keeps the flag around.
Some tests have to be removed accordingly since they were specifically
testing flag-off behaviour that is no longer true.

(effectively this is a backing-out of D2455 and D2469)

Depends on D3253

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: deadalnix, Mengerian, #bitcoin_abc

Reviewed By: deadalnix, Mengerian, #bitcoin_abc

Subscribers: jasonbcox

Maniphest Tasks: T653

Differential Revision: https://reviews.bitcoinabc.org/D3331