s390/cio: Remove unused inline function idset_sch_get_first

s390/cio: Remove unused inline function idset_sch_get_first

commit 8ebd51a705c5 ("s390/cio: idset.c: remove some unused functions")
left behind this, remove it

Link: https://lkml.kernel.org/r/20200508140643.30540-1-yuehaibing@huawei.com
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Vineeth Vijayan <vneethv@linux.ibm.com>
[vneethv@linux.ibm.com: Slight modification in the title]
Signed-off-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>