docs: iommu: Remove outdated Documentation/userspace-api/iommu.rst

The Documentation/userspace-api/iommu.rst file has become outdated due
to the removal of associated structures and APIs.

Specifically, struct such as iommu_cache_invalidate_info and guest
pasid related uapi were removed in commit 0c9f1787 ("iommu:
Remove guest pasid related interfaces and definitions").
And the corresponding uapi/linux/iommu.h file was removed in
commit 00a9bc60

 ("iommu: Move iommu fault data to
linux/iommu.h").

Signed-off-by: Mark-PK Tsai <mark-pk.tsai@mediatek.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20240702120617.26882-1-mark-pk.tsai@mediatek.com


[will: Remove stale reference to 'iommu' from index.rst]
Signed-off-by: Will Deacon <will@kernel.org>