Don't need to assert for pos.is_ok() when position is constant

It's only necessary to do the checking at the end of every non-const
member (including the constructors and from_fen()) of class Position.
Once the post-condition of every modifier guarantees the class invariant,
we don't need to verify sanity of the position as preconditions for outside
callers such as movegen, search etc. For non-class types such as Move and
Square we still need to assert of course.

Suggested by Rein Halbersma.

No functional change.

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

src/move.cpp

@@ -88,7 +88,6 @@ const string move_to_san(Position& pos, Move m) {
   if (m == MOVE_NULL)
       return "(null)";
 
-  assert(pos.is_ok());
   assert(move_is_ok(m));
 
   Bitboard attackers;