Long range pathfinder cleanup

Remove ACCEPT_DIAGONAL_GAPS as it was unused and will not be used in the future.
Use PASSABLE macro everywhere.
Simplify considerable OnTheWay as MakeGoalReachable returns a point goal.

This doubles as a very slight optimisation, on the order of microseconds per call.

Reviewed By: temple

Differential Revision: https://code.wildfiregames.com/D80