purge unused flattenable enums

... on the assumption that we don't need stable values

related: https://skia-review.googlesource.com/c/skia/+/417979

Change-Id: I483309052bc34733a8abe87fb188278d203cd6cf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/417978
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>

include/core/SkFlattenable.h

@@ -33,11 +33,7 @@ public:
         kSkMaskFilter_Type,
         kSkPathEffect_Type,
         kSkPixelRef_Type,
-        kSkUnused_Type4,    // used to be SkRasterizer
         kSkShaderBase_Type,
-        kSkUnused_Type,     // used to be SkUnitMapper
-        kSkUnused_Type2,
-        kSkUnused_Type3,    // use to be NormalSource,
     };
 
     typedef sk_sp<SkFlattenable> (*Factory)(SkReadBuffer&);