Remove mechanism to produce a big block at fork time

Summary:
The big block rule was removed, so it is all good.

Remove also the configuration constraints related to this.

Test Plan:

make check
../qa/pull-tester/rpc-tests.py

Reviewers: freetrader, CCulianu, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Subscribers: sickpig

Differential Revision: https://reviews.bitcoinabc.org/D423