Do not connect to peers that do not use the cash magic anymore.

Summary: All major implemention now are compatible with the cash magic. It does make sense to start to enforce its use rather than just accepting it.

Test Plan:

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

Run a node.

Reviewers: schancel, freetrader, Mengerian, #bitcoin_abc, dagurval

Reviewed By: Mengerian, #bitcoin_abc, dagurval

Differential Revision: https://reviews.bitcoinabc.org/D668