Move game_phase() to material.cpp

For some reason, although game phase is used
only in material, it is computed in Position.

Move computation to material, where it belongs,
and remove the useless call chain.

No functional change.

src/position.h

@@ -146,7 +146,6 @@ public:
 
   // Other properties of the position
   Color side_to_move() const;
-  Phase game_phase() const;
   int game_ply() const;
   bool is_chess960() const;
   Thread* this_thread() const;