libstdc++: Remove bogus dg-error for effective-target c++20

This test no longer has additional errors for C++20 mode, so remove the
dg-error that is now failing, and the unnecessary dg-prune-output.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/scoped_allocator/69293_neg.cc: Remove
	dg-error for c++20.

libstdc++-v3/testsuite/20_util/scoped_allocator/69293_neg.cc

@@ -47,8 +47,4 @@ test01()
   auto p = sa.allocate(1);
   sa.construct(p);  // this is required to be ill-formed
   // { dg-error "failed: .* uses_allocator is true" "" { target *-*-* } 0 }
-  // { dg-error "too many initializers for 'X'" "" { target c++2a } 0 }
 }
-
-// Needed because of PR c++/92193
-// { dg-prune-output "no matching function for call to" }