Simplify the shelter mask

The line 246 of pawns.cpp is not necessary, because we restrict the bitboards
to file_of(f) in lines 253 and 256 anyway.

No functional change.

src/pawns.cpp

@@ -240,14 +240,12 @@ Value Entry::shelter_storm(const Position& pos, Square ksq) {
 
  enum { BlockedByKing, Unopposed, BlockedByPawn, Unblocked };
 
- File center = std::max(FILE_B, std::min(FILE_G, file_of(ksq)));
- Bitboard b = pos.pieces(PAWN)
- & (forward_ranks_bb(Us, ksq) | rank_bb(ksq))
- & (adjacent_files_bb(center) | file_bb(center));
+ Bitboard b = pos.pieces(PAWN) & (forward_ranks_bb(Us, ksq) | rank_bb(ksq));
  Bitboard ourPawns = b & pos.pieces(Us);
  Bitboard theirPawns = b & pos.pieces(Them);
  Value safety = MaxSafetyBonus;
 
+ File center = std::max(FILE_B, std::min(FILE_G, file_of(ksq)));
  for (File f = File(center - 1); f <= File(center + 1); ++f)
  {
  b = ourPawns & file_bb(f);