isl_schedule_tree documentation: remove remnant of preliminary implementation
isl_schedule_tree documentation: remove remnant of preliminary implementation

The documentation of isl_schedule_tree mistakenly referred to
an aspect of a preliminary implementation that never made it
to the public version.

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

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