Revert of add SK_API to SkImageInfo (patchset #1 of https://codereview.chromium.org/513333003/)

Reason for revert:
need to revert the big ImageInfo CL but since this was landed on top of it, I need to revert this guy first.

Original issue's description:
> add SK_API to SkImageInfo
>
> BUG=skia:
> TBR=bungeman
> NOTRY=True
>
> Committed: https://skia.googlesource.com/skia/+/bedec8456ff0ea814ed1bba748a86678f458d063

TBR=
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Author: reed@google.com

Review URL: https://codereview.chromium.org/518543002

include/core/SkImageInfo.h

@@ -135,7 +135,7 @@ bool SkColorTypeValidateAlphaType(SkColorType colorType, SkAlphaType alphaType,
 /**
  *  Describe an image's dimensions and pixel type.
  */
-struct SK_API SkImageInfo {
+struct SkImageInfo {
 public:
     SkImageInfo() {}