prune unused SkNx features

prune unused SkNx features

  - remove float -> int conversion, keeping float -> byte
  - remove support for doubles

I was thinking of specializing Sk8f for AVX.  This will help keep the complexity down.

This may cause minor diffs in radial gradients: toBytes() rounds where castTrunc() truncated.  But I don't see any diffs in Gold.
https://gold.skia.org/search2?issue=1411563008&unt=true&query=source_type%3Dgm&master=false

BUG=skia:4117
CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Review URL: https://codereview.chromium.org/1411563008