Revert change to PreciousBlock() comment made in D1182

Summary:
Reverts change to commet for PreciousBlock() in validation.h that was
made in D1182. This reduces our code ownership and merge conflicts when
backporting.

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4138