TransformControls: fix bug when bind on orthographic camera

[elitecool]

when the transformcontrols bind to the orthographic camera, the eye direction is also the subtraction of camera position and object world position;

[arodic]

Honestly, I don't remember why there was a special case for PerspectiveCamera here. Your change appears to work fine. However, notice that this makes the whole if statement redundant since both branches are the same. Perhaps we should remove the whole if statement and replace it with

eye.copy( cameraPosition ).sub( worldPosition ).normalize();

[mrdoob]

I'll merge and clean up afterwards.

[mrdoob]

Thanks!