Always use the parallel check codepath for block validation

Summary: Less codepath, more fun. We also had a bug linked to only one of the two path being tested.

Test Plan: Synced the testnet with -par=0 and -par=1

Reviewers: freetrader, CCulianu, #bitcoin_abc, sickpig, dagurval

Reviewed By: #bitcoin_abc, sickpig, dagurval

Subscribers: dagurval, sickpig

Differential Revision: https://reviews.bitcoinabc.org/D521