c++: remove redundant testcase [PR83258]

I noticed only after the fact that the new testcase template/function2.C
(from r14-708-gc3afdb8ba8f183) is just a subset of ext/visibility/anon8.C,
so let's get rid of it.

	PR c++/83258

gcc/testsuite/ChangeLog:

	* g++.dg/ext/visibility/anon8.C: Mention PR83258.
	* g++.dg/template/function2.C: Removed.

(cherry picked from commit 10098788)

gcc/testsuite/g++.dg/ext/visibility/anon8.C

@@ -26,7 +26,7 @@ int main ()
     static void fn2 () {}
   };
   call<&B1::fn1> ();
-  call<&B2::fn2> ();	// { dg-error "linkage|no matching" "" { target c++14_down } }
+  call<&B2::fn2> ();	// { dg-error "linkage|no matching" "PR83258" { target c++14_down } }
   call<&fn3> ();
   call<&B1::fn4> ();
   call<&fn5> ();	// { dg-error "linkage|no matching" "" { target { ! c++11 } } }

gcc/testsuite/g++.dg/template/function2.C

-// PR c++/83258
-
-template<void(*)()> struct A { };
-
-int main() {
-  struct B { static void f() { } };
-  A<B::f> a; // { dg-error "linkage" "" { target c++14_down } }
-}