Remove experimental::Accessor

[bernhardmgruber]

We added alpaka::experimental::Accessor and some related classes to support SYCL as a backend for SYCL versions without pointers to device memory on the host (USM). Since #1482 was merged, this functionality can just as well be provided by std::mdspan. Furthermore, work is currently ongoing on reengineering the SYCL backend to require USM and thus also make alpaka::experimental::Accessor obsolete. See #1845.

Depending on whether USM will be required for the SYCL backend, we may need to add specializations for alpaka::GetMdspan to create mdspans from SYCL buffers/accessors, or it may just work out of the box with USM pointers. In both cases, alpaka::experimental::Accessor is no longer required.

I would suggest we wait for the USM reengineering to finish and then remove alpaka::experimental::Accessor.

Depends on:

• Rewrite the SYCL backend for the SYCL 2020 standard and USM allocations #1845