Merge #11880: Stop special-casing phashBlock handling in validation for TBV

Summary:
9c5a4a6ed Stop special-casing phashBlock handling in validation for TBV (Matt Corallo)

Pull request description:

There is no reason to do this, really, we already have "ignore PoW" flags. Motivated by https://github.com/bitcoin/bitcoin/pull/11739#discussion_r155841721

Tree-SHA512: 37cb1ae5b11c9e8ed7a679bb07ad3b119a2a014744b26d197d67ba21beb19fe6815271df935e40f7c7bd5f2e4d7ae4dad7bd4d00fa230a8d789f37e9de31a769

Backport of Core PR 11880
https://github.com/bitcoin/bitcoin/pull/11880/files

Test Plan:
ninja check
make check
test_runner.py

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2898