GS:MTL: Don't override GSTexture::Save with a TODO

There's a working default implementation

pcsx2/GS/Renderers/Metal/GSTextureMTL.h

@@ -74,7 +74,6 @@ public:
 	void* MapWithPitch(const GSVector4i& r, int pitch, int layer);
 	void Unmap() override;
 	void GenerateMipmap() override;
-	bool Save(const std::string& fn) override;
 	void Swap(GSTexture* tex) override;
 	id<MTLTexture> GetTexture() { return m_texture; }
 };

pcsx2/GS/Renderers/Metal/GSTextureMTL.mm

@@ -189,12 +189,6 @@ void GSTextureMTL::GenerateMipmap()
 	}
 }}
 
-bool GSTextureMTL::Save(const std::string& fn)
-{
-	// TODO: Implement
-	return false;
-}
-
 void GSTextureMTL::Swap(GSTexture* other)
 {
 	GSTexture::Swap(other);