Simplify evaluate_scale_factor()

Code coverage with gcob shows that this path
is almost never taken, so try to simplify it
out.

bench: 5415646

src/evaluate.cpp

@@ -781,17 +781,8 @@ namespace {
     if (sf == SCALE_FACTOR_NORMAL || sf == SCALE_FACTOR_ONEPAWN)
     {
         if (pos.opposite_bishops())
-        {
-            // Endgame with opposite-colored bishops and no other pieces (ignoring pawns)
-            // is almost a draw, in case of KBP vs KB, it is even more a draw.
-            if (   pos.non_pawn_material(WHITE) == BishopValueMg
-                && pos.non_pawn_material(BLACK) == BishopValueMg)
-                return more_than_one(pos.pieces(PAWN)) ? ScaleFactor(31) : ScaleFactor(9);
-
-            // Endgame with opposite-colored bishops, but also other pieces. Still
-            // a bit drawish, but not as drawish as with only the two bishops.
             return ScaleFactor(46);
-        }
+
         // Endings where weaker side can place his king in front of the opponent's
         // pawns are drawish.
         else if (    abs(eg) <= BishopValueEg