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)