Delete RenderEngine unused code

* V0 skia implementation beat the prior change deleting unused code, so
delete bindExternalTextureBuffer there.
* Remove useNativeFenceSync, now that there's no more usages
* Remove impl::RenderEngine, since there's no need for a private
impl-superclass spanning every RenderEngine implementation
* Clean up mock/RenderEngine

Bug: 168553972
Test: builds
Change-Id: I6f760510656b90b30d86b04fc70c19067a4491cd
diff --git a/libs/renderengine/skia/SkiaGLRenderEngine.h b/libs/renderengine/skia/SkiaGLRenderEngine.h
index eb098cb..b8387b2 100644
--- a/libs/renderengine/skia/SkiaGLRenderEngine.h
+++ b/libs/renderengine/skia/SkiaGLRenderEngine.h
@@ -36,8 +36,8 @@
 class SkiaGLRenderEngine : public skia::SkiaRenderEngine {
 public:
     static std::unique_ptr<SkiaGLRenderEngine> create(const RenderEngineCreationArgs& args);
-    SkiaGLRenderEngine(const RenderEngineCreationArgs& args, EGLDisplay display, EGLConfig config,
-                       EGLContext ctxt, EGLSurface placeholder, EGLContext protectedContext,
+    SkiaGLRenderEngine(EGLDisplay display, EGLConfig config, EGLContext ctxt,
+                       EGLSurface placeholder, EGLContext protectedContext,
                        EGLSurface protectedPlaceholder);
     ~SkiaGLRenderEngine() override{};