Remove old workarounds; Fix build with current hipSYCL

[fknorr]

• Removes the ComputeCpp CMake integration
• Removes all #preprocessor workarounds for ComputeCpp
• Bumps the minimum hipSYCL version to d2bd9fc7 to follow the get_info API change from enum parameters to type parameters (see [SYCL2020] Migrate information descriptors to their respective namespaces AdaptiveCpp/AdaptiveCpp#987) and removes all workarounds for older versions of hipSYCL
• Removes the subscript_result_t workaround for ComputeCpp's Clang-8 buggy implementation of decltype(auto)
• Transitions all examples to unnamed kernels (kernel names were only required for CCPP)

This allows building Celerity with the latest hipSYCL (The latest image is not tagged yet due to a CI bug, so 23.04 appears broken even though it passes on a manual trigger).

Unfortunately the DPC++ CI is still broken due to what appears to be UB in the Intel IGC we've recently upgraded to. At some point we should bisect IGC versions to find a working revision and downgrade both our Docker environments and CI runner hosts accordingly.

Partially addresses celerity/meta#20, but some non-trivial workarounds remain that were introduced because of CCPP.

[github-actions]

Check-perf-impact results: (9e900a306dc9f17e4a27439205a7680c)

❓ No new benchmark data submitted. ❓
Please re-run the microbenchmarks and include the results if your commit could potentially affect performance.

[PeterTh]

LGTM, other than one minor remark regarding the examples.
Always nice to get rid of a lot of obsolete code.

[psalz]

Feels good!