Delete dead code related to AVX

[danmoseley]

Some of the original code relating to AVX and SSE is dead and can be deleted including

• all of avx.cpp and avx.cs
• functions in thunk.cs that exposed avx.cpp functions, eg MatMulDX

I checked in TLC as well and the above were only used by code in NeuralNetworks/*, which will not migrate to ML.NET.

I also notice some functions in sse.cs seem dead eg MeanOfSrc

[Zruty0]

@yaeldekel , do you have an objection? If not, we'll take it as a bug.

[jwood803]

@danmosemsft Is this still an issue? I looked through the code and I'm not seeing the dead code. I could just be missing it, though, but thought I would double check. 😄

[Anipik]

I removed all of the dead code from cpumath library