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/NativeVertexFormat.h

@@ -108,7 +108,6 @@ class NativeVertexFormat : NonCopyable
 public:
 	virtual ~NativeVertexFormat() {}
 
-	virtual void Initialize(const PortableVertexDeclaration &vtx_decl) = 0;
 	virtual void SetupVertexPointers() = 0;
 
 	u32 GetVertexStride() const { return vtx_decl.stride; }