docs: iommu: Remove outdated Documentation/userspace-api/iommu.rst

The Documentation/userspace-api/iommu.rst file has become outdated due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 0c9f178 ("iommu:
Remove guest pasid related interfaces and definitions").
And the corresponding uapi/linux/iommu.h file was removed in
commit 00a9bc6 ("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>

Author: PeikanTsai

Documentation/userspace-api/index.rst

@@ -44,7 +44,6 @@ Devices and I/O
    accelerators/ocxl
    dma-buf-alloc-exchange
    gpio/index
-   iommu
    iommufd
    media/index
    dcdbas

Documentation/userspace-api/iommu.rst

@@ -11543,7 +11543,6 @@ L:	iommu@lists.linux.dev
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/joro/iommu.git
 F:	Documentation/devicetree/bindings/iommu/
-F:	Documentation/userspace-api/iommu.rst
 F:	drivers/iommu/
 F:	include/linux/iommu.h
 F:	include/linux/iova.h