BGE cleanup: Removing the PHY__Vector classes and replacing them with MT_Vectors. The PHY__Vectors didn't offer anything (not even any real abstraction) and they required annoying MT_Vector <-> PHY_Vector conversions all over the place. No functional changes.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54722 954f8c5b-7b00-dc11-b283-0030488c597c