always unpark even when -parkdeepreorg=0

Summary:
The parked / parkedparent flags do get saved in the block index,
and so if we restart with -parkdeepreorg=0 we don't want to get
stuck in a situation where a chain gets permanently parked until
the user manually intervenes with unparkblock RPC.

Test Plan: ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5027