Remove duplicate implementations of (setf aref/sbit/bit).

Since (setf aref/sbit/bit) have to work with
(setf (apply #'aref array subscripts)), they had both a setf expander
and a setf-function, but it can be implemented with just a
setf-function. All other accessors are still done using
(defsetf accessor %setaccessor), I haven't found a technical reason to
prefer one to another, other than (setf accessor) being a nicer name.

Fixes lp#1241095.