drop isl_map_{lower,upper}_bound_multi_val

drop isl_map_{lower,upper}_bound_multi_val

These functions were introduced together with
isl_set_{lower,upper}_bound_multi_val and
isl_{set,map}_{lower,upper}_bound_multi_pw_aff in
isl-0.22.1-85-gedc267ae08 (add exported
isl_{set,map}_{lower,upper}_bound_multi_{val,pw_aff},
Tue Oct 16 11:30:24 2018 +0200).
However, they require the bound to live in the same space
as the map.  This makes sense for the other variants,
but it is difficult and not all that meaningful
to create an isl_multi_val in a map space.

Drop these functions for now.  They were introduced after
the latest release and they have presumably never been used.

Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>