Use stronglyProtected

~stronglyProtected is quite similar to ~attackedBy[Them][PAWN] & ~attackedBy2[Them],
the only difference appears to be that the former includes squares attacked twice
by both sides. The resulting logic is simpler, and the change appears to be at least
Elo-neutral at both STC and LTC.

STC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 35924 W: 7978 L: 7885 D: 20061
http://tests.stockfishchess.org/tests/view/5c14a5c00ebc5902ba11ed72

LTC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 37078 W: 6125 L: 6030 D: 24923
http://tests.stockfishchess.org/tests/view/5c14ae880ebc5902ba11eed8

Bench: 3646542

src/evaluate.cpp

@@ -560,8 +560,7 @@ namespace {
 
  // Bonus for restricting their piece moves
  restricted = attackedBy[Them][ALL_PIECES]
- & ~attackedBy[Them][PAWN]
- & ~attackedBy2[Them]
+ & ~stronglyProtected
  & attackedBy[Us][ALL_PIECES];
  score += RestrictedPiece * popcount(restricted);