Revert "impl gather8/gather16 with gather32"

This reverts commit d4e3b9e8bcfc5e4cd4ea0b587a3b0cb76f32d0c0.

Reason for revert: will reland with fixes

Original change's description:
> impl gather8/gather16 with gather32
> 
> This is our quick path to JIT small gathers.
> 
> The idea is roughly,
> 
>    const uint32_t* ptr32 = ptr8;
>    uint32_t abcd = ptr32[ix/4];
>    switch (ix & 3) {
>      case 3: return (abcd >> 24)       ;
>      case 2: return (abcd >> 16) & 0xff;
>      case 1: return (abcd >>  8) & 0xff;
>      case 0: return (abcd      ) & 0xff;
>    }
> 
> With the idea that if we may load a given byte,
> we should also be allowed to load the four byte
> aligned word that byte falls within.
> 
> Change-Id: I7fb1085306050c918ccf505f1d2e1e87db3b8c9a
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/268381
> Reviewed-by: Herb Derby <herb@google.com>
> Commit-Queue: Mike Klein <mtklein@google.com>

TBR=mtklein@google.com,herb@google.com,reed@google.com

Change-Id: I48d800edc6517f37e04752c91616b666a5e0f384
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/268490
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed