Usings are just shortcut definitions for type naming (class / struct / enum / delegate) ... so you can use the type names directly under the namespaces indicated by usings:
using System;
// agora será possível usar a classe String sem especificar o namespace
class Xpto
{
public String Nome { get; set; }
}
When removing a using, what is being done is that the shortcut can no longer be used, but it is still possible to use the full name to refer to the type:
// sem o using temos que usar o namespace junto do nome
// mas ainda assim a classe pode ser referenciada
class Xpto
{
public System.String Nome { get; set; }
}
Both of the above forms are identical in the compiled final result.