[qa] change bip68 sequence to use calculate_fee

Summary: As per title

Test Plan:

./test/functional/test_runner.py bip68-sequence

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: jasonbcox, deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1561