C API: remove dead code, simplify boilerplate

gm/cgms.cpp

Index: gm/cgms.cpp
diff --git a/gm/cgms.cpp b/gm/cgms.cpp
index e3e84b03e111d3addafcf9d19d0ca513928e078f..832a7b50fd741b3fd9e2c9d6b2eeb8064fbe5ec0 100644
--- a/gm/cgms.cpp
+++ b/gm/cgms.cpp
@@ -10,26 +10,6 @@
 
 extern "C" void sk_test_c_api(sk_canvas_t*);
 
-class C_GM : public skiagm::GM {
-public:
-    C_GM() {}
-
-protected:
-    SkString onShortName() override {
-        return SkString("c_gms");
-    }
-
-    SkISize onISize() override {
-        return SkISize::Make(640, 480);
-    }
-
-    void onDraw(SkCanvas* canvas) override {
-        sk_test_c_api((sk_canvas_t*)canvas);
-    }
-
-private:
-    typedef GM INHERITED;
-};
-
-DEF_GM( return new C_GM; )
-
+DEF_SIMPLE_GM(c_gms, canvas, 640, 480) {
+    sk_test_c_api((sk_canvas_t*)canvas);
+}