Remove CChainParams parameter from BlockAssembler

Summary:
Remove CChainParams parameter from BlockAssembler

Fixes T169

Test Plan: make check

Reviewers: jasonbcox, deadalnix, #bitcoin_abc, schancel

Reviewed By: jasonbcox, #bitcoin_abc, schancel

Subscribers: teamcity

Maniphest Tasks: T169

Differential Revision: https://reviews.bitcoinabc.org/D1031