Remove Position.hasCastlingExecuted

[lrozenblyum]

This flag was added during #293 in order to make CastlingSafety work correctly.
However it's not needed in Position class logic itself.
Such historical information may be kept by storing reference of Position to Position (and/or maybe Move history).

This historical overview may be also needed during #158 #155 so this task may be done in combination with one of them.