Remove (almost) unused read/writeFunctionPtr API.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1937053002

Review-Url: https://codereview.chromium.org/1937053002
diff --git a/tests/ImageFilterTest.cpp b/tests/ImageFilterTest.cpp
index 56b3aaf..592f5c6 100644
--- a/tests/ImageFilterTest.cpp
+++ b/tests/ImageFilterTest.cpp
@@ -65,9 +65,7 @@
     }
 
     void flatten(SkWriteBuffer& buffer) const override {
-        this->INHERITED::flatten(buffer);
-        buffer.writeFunctionPtr(fReporter);
-        buffer.writeMatrix(fExpectedMatrix);
+        SkDEBUGFAIL("Should never get here");
     }
 
 private:
@@ -289,11 +287,8 @@
 }
 
 sk_sp<SkFlattenable> MatrixTestImageFilter::CreateProc(SkReadBuffer& buffer) {
-    SK_IMAGEFILTER_UNFLATTEN_COMMON(common, 1);
-    skiatest::Reporter* reporter = (skiatest::Reporter*)buffer.readFunctionPtr();
-    SkMatrix matrix;
-    buffer.readMatrix(&matrix);
-    return MatrixTestImageFilter::Make(reporter, matrix);
+    SkDEBUGFAIL("Should never get here");
+    return nullptr;
 }
 
 #ifndef SK_IGNORE_TO_STRING