Remove redundant items from STANDARD_SCRIPT_VERIFY_FLAGS

Summary:
SCRIPT_VERIFY_LOW_S and SCRIPT_VERIFY_NULLFAIL are already included
as part of MANDATORY_SCRIPT_VERIFY_FLAGS.

Part of T653

Test Plan:
make check
./test/functional/test_runner.py

Reviewers: deadalnix, markblundeberg, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Maniphest Tasks: T653

Differential Revision: https://reviews.bitcoinabc.org/D3059