Two 2D arrays were passed to HandleInterMode and not used elsewhere.

One has been added to the HandleInterModeArgs struct to
simplify handle_inter_mode's signature and the the calling function.

The other has been unread since commit
beacb3ff. That patch removed the
"skip_txfm optimization" but left the single_skippable array in place,
even though it was no longer read. Since the array's only usage has been
removed, this patch removes the variable outright.

Change-Id: I6e680a9ddc6f67961153819a62009751252757a8