[temp.mem.enum] Remove instantiation in example [temp.inst] (#7558)

The example is inconsistent with [temp.inst]p3. Since the implicit instantiation
does not contribute to the point of [temp.mem.enum], the inconsistency
can be resolved by omitting the instantiation.

Author: thebrandre

source/templates.tex

@@ -2654,7 +2654,6 @@
 template<class T> struct A {
   enum E : T;
 };
-A<int> a;
 template<class T> enum A<T>::E : T { e1, e2 };
 A<int>::E e = A<int>::e1;
 \end{codeblock}