Replace Axion activation time with the actual block height

Summary:
The axion upgrade activated long time ago already, cleanup is long overdue.

This diff changes the activation from MTP based to height based, remove the -axionactivationtime option and updates the tests accordingly. Since we don't need to test for activation anymore, the regtest activation height is set to 0.

Depends on D11080.

Test Plan:

ninja all check-all

Run IBD with no assume valid.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11092