Remove MOUSE_MOVE pathfinding

Allow me to explain my reasoning for this:

With pathfinding on, player movement was very "jittery" and generally disorienting. This is because holding down the mouse button would continually recalculate the path to new endpoints as the camera moved around. 

Pathfinding like this is great for movement systems where a mouse click will set a waypoint, and that waypoint won't change until it is reached (or a cooldown timer finishes). However, Flare's movement system is closer to holding down a "run" button than it is to setting waypoints.