Remove friendship of deleted old class GrTextBlob

Change-Id: I27eb99450b2b261a4688c62d91a9002dfe7d14df
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/548484
Auto-Submit: Herb Derby <herb@google.com>
Commit-Queue: Herb Derby <herb@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>

include/core/SkTextBlob.h

@@ -251,7 +251,6 @@ private:
     }
 
     friend class SkGlyphRunList;
-    friend class GrTextBlobCache;
     friend class SkTextBlobBuilder;
     friend class SkTextBlobPriv;
     friend class SkTextBlobRunIterator;