Replace phonon activation time with the height it activated at

Summary:
Now that the phonon upgrade is activated, we can replace the
activation time with the height it activated at.

Test Plan: ninja check check-functional

Reviewers: #bitcoin_abc, majcosta, Fabien

Reviewed By: #bitcoin_abc, majcosta, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7177