Remove unused struct.

TBR=mtklein@google.com

Change-Id: Iad5cda6bc9b9728ade249e8e88ae830798955a3e
Reviewed-on: https://skia-review.googlesource.com/7339
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Herb Derby <herb@google.com>

src/core/SkArenaAlloc.cpp

@@ -39,10 +39,6 @@ char* SkArenaAlloc::NextBlock(char* footerEnd) {
     return nullptr;
 }
 
-struct Skipper {
-    char* operator()(char* objEnd, uint32_t size) { return objEnd - size; }
-};
-
 SkArenaAlloc::SkArenaAlloc(char* block, size_t size, size_t extraSize)
     : fDtorCursor {block}
     , fCursor     {block}