Revert of add dummy avx file so xcode will build (patchset #1 id:1 of https://codereview.chromium.org/916563002/)

Reason for revert:
Going to punt on 16-bit float support for now.  Can't figure out ARM 64.

Original issue's description:
> add dummy avx file so xcode will build
>
> BUG=skia:
>
> Committed: https://skia.googlesource.com/skia/+/a0921f2563701d54e4e022de99f2705f4ada8a6e

TBR=reed@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/912213002

gyp/opts.gypi

@@ -84,6 +84,5 @@
             '<(skia_src_path)/opts/SkBlitRow_opts_SSE4.cpp',
         ],
         'avx_sources': [
-            '<(skia_src_path)/opts/SkDummy_opts_AVX.cpp',
         ],
 }

src/opts/SkDummy_opts_AVX.cpp

@@ -1,2 +0,0 @@
-
-// This file intentionally left blank