Simplify ThreatBySafePawn evaluation

Simplify ThreatBySafePawn evaluation

Simplify ThreatBySafePawn evaluation by removing the 'if (weak)' speed
optimization check from threats evaluation. This is a non functional
change as it removes just a speed optimization conditional which was
probably useful before but does no longer provide benefits. This section
section had a few more lines not long ago, with ThreatByHangingPawn and
a loop through the threatened pieces, but now there is not much left.

Passed STC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 47775 W: 9696 L: 9624 D: 28455
http://tests.stockfishchess.org/tests/view/5ac298910ebc591746423f8b

Closes https://github.com/official-stockfish/Stockfish/pull/1531

Non functional change.