[roll] Roll fuchsia [kernel][iommu] Remove the Intel IOMMU implementation.

The Intel IOMMU implementation has been around for years, but only in
a half-finished state.  It was never really made production ready, nor
enabled in a production environment.

As progress has been getting make in standing up support for ARM SMMUs
(v2 at least), this has started to become problematic.  There are
parts of the layering between the generic dispatcher level and the
specific kernel driver level which need some updating and refactoring,
and it is extremely difficult to do this in a way where we have
confidence that the Intel driver level will continue "working".

The reality is that it was a project under in development when it got
back-burnered approximately 7 years ago.  There are no real ways to
test that the implementation is still correct, especially given that
it was not fully correct even when it was shelved.

For now, remove the code to make it easier to make progress on ARM
hardware bringup.  When the day arrives that x64 is a top priority
again, we can start again, but this time hopefully working in a
framework where both the stub driver and the ARM SMMU driver exist and
have real-world deployments.  The code will still be there in git if
we want to go back to it, either to salvage pieces or to just serve as
a reference.

Original-Bug: 42108551
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1519891
Original-Revision: 0b2e904a938145f6b422f7091fd1c8519bcb7ee6
GitOrigin-RevId: 3097e1c51d8c5ce7e5629ad5f8c33ce70222592d
Change-Id: I2b61e18c9e4428674bac598e155f9ed39c041aaf