remove SK_IGNORE_TO_STRING

SK_IGNORE_TO_STRING is not defined anywhere.
The same effect can be had by using a modern
linker.

Removing it simplifies bookmaker and makes
our includes easier to understand.

R=robertphillips@google.com
TBR=reed@google.com

Bug: skia:6898
Change-Id: Ib716f5ef1b42a7fbda0df43ece212d1b7c40289f
Reviewed-on: https://skia-review.googlesource.com/118963
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Cary Clark <caryclark@skia.org>
Commit-Queue: Cary Clark <caryclark@skia.org>
diff --git a/include/core/SkDrawLooper.h b/include/core/SkDrawLooper.h
index 9012802..e318e11 100644
--- a/include/core/SkDrawLooper.h
+++ b/include/core/SkDrawLooper.h
@@ -99,7 +99,7 @@
      */
     virtual bool asABlurShadow(BlurShadowRec*) const;
 
-    SK_TO_STRING_PUREVIRT()
+    virtual void toString(SkString* str) const = 0;
     SK_DEFINE_FLATTENABLE_TYPE(SkDrawLooper)
 
 protected: