drop isl_set_plain_dim_is_fixed
drop isl_set_plain_dim_is_fixed

This function was introduced in 9ddd482 (add isl_set_fast_dim_is_fixed and
isl_map_fast_input_is_fixed, Thu Sep 25 10:07:39 2008 +0200) without
ever being used.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

PollyLabs/isl,PollyLabs/isl,nicolasvasilache/isl,Meinersbur/isl,inducer/isl-mirror,inducer/isl-mirror,PollyLabs/isl,tobig/isl,Meinersbur/isl,nicolasvasilache/isl,nicolasvasilache/isl,tobig/isl,PollyLabs/isl,tobig/isl,inducer/isl-mirror,tobig/isl,inducer/isl-mirror,Meinersbur/isl,PollyLabs/isl,inducer/isl-mirror,nicolasvasilache/isl,Meinersbur/isl,nicolasvasilache/isl