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/evaluate.cpp

@@ -275,7 +275,6 @@ Value do_evaluate(const Position& pos, Value& margin) {
   Value margins[2];
   Score score, mobilityWhite, mobilityBlack;
 
-  assert(pos.is_ok());
   assert(pos.thread() >= 0 && pos.thread() < MAX_THREADS);
   assert(!pos.in_check());