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