[namespace.udecl] Remove incorrect example.

CWG36 (still open) notes that this example is incorrect, but was left in
the standard, confusing people for 17 years. Remove the example rather
than fixing it to match the normative text, because the validity of this
code is still under dispute.

Author: zygoloid

source/declarations.tex

@@ -2656,11 +2656,6 @@
   using A::i;       // OK: double declaration
 }
 
-void f() {
-  using A::i;
-  using A::i;       // error: double declaration
-}
-
 struct B {
   int i;
 };