Remove HVX extension support

[ermig1979]

Summary

Removes the remaining Hexagon HVX code path from the library, now that the earlier function-level HVX optimizations have already been dropped.

• Deletes the last HVX implementation (HistogramConditional) together with HVX load/store/compare helpers, alignment, CPU enablement, and the SIMD_HVX_ENABLE / SIMD_HVX_DISABLE / SIMD_HVX_FUNC macros.
• Removes SimdCpuInfoHvx and the matching Python Simd.CpuInfo.HVX value, and shifts CurrentFrequency to the next index.
• Drops HVX from the test framework (auto-test registration, -de help text, C++ CPU info printout, and the performance report column).
• Removes the Visual Studio 2022 Hvx project, the Hexagon CMake target, and the build_and_test_hexagon_cross GitHub Actions job.
• Updates README, the site description, generated help, and the 7.2.166 release notes.

Test plan

• Release build with g++ (SIMD_SHARED=ON, AVX-512VNNI, AMX-BF16)
• ./Test "-r=.." -m=a -fi=HistogramConditional -tt=1 -ts=1 -mt=1 -cc=1 — all tests passed; CPU info and the performance report no longer mention HVX
• Python Simd.Lib.SysInfo() reports current frequency and has no Simd.CpuInfo.HVX value
• Remaining HVX mentions are only the 7.2.166 removal notes and the historical 7.0.160 release notes