Retire PawnsFileSpan

It is useless. Tested as no regression:

STC
LLR: 4.06 (-2.94,2.94) [-3.00,1.00]
Total: 140718 W: 28527 L: 28568 D: 83623

LTC
LLR: 2.94 (-2.94,2.94) [-3.00,1.00]
Total: 60034 W: 10359 L: 10303 D: 39372

bench: 6564212

Resolves #88

src/pawns.cpp

@@ -57,9 +57,6 @@ namespace {
     S( 0, 0), S( 0, 0), S(0, 0), S(0, 0),
     S(20,20), S(40,40), S(0, 0), S(0, 0) };
 
-  // Bonus for file distance of the two outermost pawns
-  const Score PawnsFileSpan = S(0, 8);
-
   // Unsupported pawn penalty
   const Score UnsupportedPawnPenalty = S(20, 10);
 
@@ -183,10 +180,6 @@ namespace {
     b = e->semiopenFiles[Us] ^ 0xFF;
     e->pawnSpan[Us] = b ? int(msb(b) - lsb(b)) : 0;
 
-    // In endgame it's better to have pawns on both wings. So give a bonus according
-    // to file distance between left and right outermost pawns.
-    value += PawnsFileSpan * e->pawnSpan[Us];
-
     return value;
   }