btrfs: remove unused variable from setup_cluster_no_bitmap

The variable window_start in setup_cluster_no_bitmap is not used since commit
1bb91902


(Btrfs: revamp clustered allocation logic)

Signed-off-by: Valentina Giusti <valentina.giusti@microon.de>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <clm@fb.com>