Should we keep or remove the un-used system namespaces in C# class ?

I have a very basic question about the system namespaces. I have noticed when you create a new C# class the system namespaces are already included example -

using System;
using System.Text;

I am not sure if they are not used, then should we keep them or remove them. I don't see any standard documentation around it also I see mixed answers over internet. Please advise on this.