Remove unused great wall activation code

Summary:
Like monolith upgrade, this one was possible to make retroactively
always active. After this change, "great wall" is only mentioned in
the chainparams.cpp checkpoint comment.

Depends on D3253 and D3194

Test Plan:
make check
test_runner.py (to show nothing was using -greatwallactivationtime)

Reviewers: jasonbcox, Fabien, Mengerian, #bitcoin_abc, deadalnix

Reviewed By: Mengerian

Maniphest Tasks: T653

Differential Revision: https://reviews.bitcoinabc.org/D3255