I don't believe there are arrows to be tweaked, hidden or otherwise; they were done away with in Lion.
A perusal of
TinkerTool 4: Details reveals that "Place both scroll arrow buttons of any scroll bar at both ends of the bar" is available for OS X 10.6, but not for 10.7 or 10.8.
John Siracusa brought his usual thorough analysis to the radical departure from Snow Leopard and earlier scrolling behavior represented by Lion (and later—some of these changes were subsequently refined) in
Mac OS X 10.7 Lion: the Ars Technica review.
