isl_map_convex_hull: handle unbounded, but pointed, case using wrapping

isl_map_convex_hull: handle unbounded, but pointed, case using wrapping

The case where the convex hull may be unbounded (but still pointed)
cannot be handled directly using wrapping.  However, by changing
the homogeneous direction, we can in such cases always transform
the input polyhedra to polytopes, compute the hull and then transform back.

Since we already handle the case where the convex hull may not be
pointed, we can now handle all cases using wrapping.
The convex_hull_pair_elim function is therefore no longer used.
We leave it in, because we may want to allow the user to choose
in future which convex hull algorithm to use.