Revert of Work around exasperating Google3 Android build error. (patchset #1 id:1 of https://codereview.chromium.org/1649493005/ )

Reason for revert:
This workaround no longer seems to be necessary.

Original issue's description:
> Work around exasperating Google3 Android build error.
>
> BUG=skia:
> GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1649493005
>
> Committed: https://skia.googlesource.com/skia/+/900901fb1ba5170549670829fc8f9d9b590ff5f0

TBR=mtklein@google.com,reed@google.com
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=skia:

Review URL: https://codereview.chromium.org/1669663003

include/core/SkXfermode.h

@@ -263,9 +263,6 @@ private:
         kModeCount = kLastMode + 1
     };
 
-    // TODO(benjaminwagner): This is crazy, but it fixes the Google3 Android build. :-(
-    virtual void doNothing() const;
-
     typedef SkFlattenable INHERITED;
 };
 

src/core/SkXfermode.cpp

@@ -1070,5 +1070,3 @@ bool SkXfermode::IsOpaque(const SkXfermode* xfer, SrcColorOpacity opacityType) {
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_START(SkXfermode)
     SK_DEFINE_FLATTENABLE_REGISTRAR_ENTRY(SkProcCoeffXfermode)
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_END
-
-void SkXfermode::doNothing() const {}