NativeVertexFormat: Inline Initialize in contructor

They were only called at once, so no need to seperate them.

This also removes the only dereference of the NativeVertexFormat in VideoCommon, so backends may just return nullptr.

Source/Core/VideoCommon/VertexManagerBase.h

@@ -50,7 +50,7 @@ public:
 
 	static void Flush();
 
-	virtual ::NativeVertexFormat* CreateNativeVertexFormat() = 0;
+	virtual NativeVertexFormat* CreateNativeVertexFormat(const PortableVertexDeclaration& vtx_decl) = 0;
 
 	static void DoState(PointerWrap& p);