s390/iommu: get rid of S390_CCW_IOMMU and S390_AP_IOMMU

s390/iommu: get rid of S390_CCW_IOMMU and S390_AP_IOMMU

These don't do anything anymore, the only user of the symbol was
VFIO_CCW/AP which already "depends on VFIO" and VFIO itself selects
IOMMU_API.

When this was added VFIO was wrongly doing "depends on IOMMU_API" which
required some contortions like this to ensure IOMMU_API was turned on.

Reviewed-by: Eric Farman <farman@linux.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/0-v2-eb322ce2e547+188f-rm_iommu_ccw_jgg@nvidia.com
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>