sched/fair: remove useless variable in find_best_target

Patch 5680f23f ("sched/fair: streamline find_best_target
heuristics") has reworked function find_best_target, as result the
variable "target_util" is useless now. So remove it.

Change-Id: I5447062419e5828a49115119984fac6cd37db034
Signed-off-by: Leo Yan <leo.yan@linaro.org>