Revert "Compute the real-time hash target for the next block"

Summary:
This reverts commit rABCccfbdd5df5fe8815b86fc98bd8e163be51dfcd6f.

I landed a previous version by mistake, let's revert this.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D16772