Remove an useless assert in evaluate_passed_pawns()

The tested square comes from a bitboard anded with
pos.pieces_of_color(Us), so assert is useless.

Another nitpick report by Marek Kwiatkowski  ;-)

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/evaluate.cpp

@@ -812,7 +812,6 @@ namespace {
     {
         Square s = pop_1st_bit(&b);
 
-        assert(pos.piece_on(s) == piece_of_color_and_type(Us, PAWN));
         assert(pos.pawn_is_passed(Us, s));
 
         int r = int(relative_rank(Us, s) - RANK_2);