Get rid of SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_WITNESS_PROGRAM

Summary: as per title

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

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1446