libvpx

Force ARNR filtering to be centered on the ARF frame

ARNR filtering is now forced to be centered on the ARF
frame and the other two options have been removed.

The other modes of constructing the ARNR frame were
not used and there does not seem to be any good
reason to maintain them.

This is purely an encoder-side change.

Change-Id: Ic772636d23f280752973852b9740083532a49de2