remove SkThreadedBMPDevice and friends

It is unused, is becoming a maintainence burden and source of bugs,
and takes up a lot of time on the *SAN bots.

Change-Id: If383eb6e4838ca23140f9e16d518b1bfc655fa12
Reviewed-on: https://skia-review.googlesource.com/143307
Auto-Submit: Mike Klein <mtklein@google.com>
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Herb Derby <herb@google.com>
diff --git a/dm/DMSrcSink.h b/dm/DMSrcSink.h
index 86c2279..d42c186 100644
--- a/dm/DMSrcSink.h
+++ b/dm/DMSrcSink.h
@@ -441,9 +441,8 @@
     Error draw(const Src&, SkBitmap*, SkWStream*, SkString*) const override;
     const char* fileExtension() const override { return "png"; }
     SinkFlags flags() const override { return SinkFlags{ SinkFlags::kRaster, SinkFlags::kDirect }; }
-protected:
-    void allocPixels(const Src& src, SkBitmap*) const;
 
+private:
     SkColorType         fColorType;
     sk_sp<SkColorSpace> fColorSpace;
 };