Simplify cmh pruning by removing PvNode exception
STC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 9935 W: 2330 L: 2184 D: 5421
http://tests.stockfishchess.org/tests/view/
5b587dc00ebc5902bdb88424
LTC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 20635 W: 3585 L: 3464 D: 13586
http://tests.stockfishchess.org/tests/view/
5b58910a0ebc5902bdb885b9
Closes https://github.com/official-stockfish/Stockfish/pull/1711
Bench:
4905530