Cleanup pre-fork testing for bip68-sequence

Summary: Removed the pre-fork configuration.

Test Plan: test_runner.py bip68-sequence (ran multiple times)

Reviewers: deadalnix, schancel, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2087