Remove deprecated SIMD support

The following platforms and SIMD layers were deprecated in !938 (merged) (which was in release 2021) after discussion at #3818 (closed) and should be removed:

• ARM Neon (ie. impl_arm_neon, not impl_arm_neon_asimd)
• Power 7 (ie. impl_ibm_vmx)
• x86 MIC (ie. KNC support in impl_86_mic; note this does not affect support on KNL)
• Sparc64 HPC ACE (ie. impl_sparc64_hpc_ace; the architecture of the former K computer)
• Decide future of tree reduction used for MIC
• Remove tree reduction used for MIC

Related cmake detection and code paths should also go away. They're not tested in CI, so nothing changes there.

There's a fancy tree reduction for MIC that could go away, which might be desirable because it is implemented using stuff from thread-MPI, whereas these days one would implement it with something like the std::atomc_flag<ptrdiff_t> or (C++20) std::latch.