Remove magnetic anomaly transitional code, activate on block height

Summary:
This include the -magneticanomalyactivationtime parameter and some
functional tests.

Depends on D2324

Test Plan:

make check
./test/functional/test_runner.py

Run IBD with checkpoint disabled and assumevalid=0 on mainnet and testnet

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2318