libstdc++: Remove unnecessary dg-prune-output from tests

libstdc++: Remove unnecessary dg-prune-output from tests

There are no errors matching this pattern in these tests (only in the
deque/48101_neg.cc and vector/48101_neg.cc tests).

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/forward_list/48101_neg.cc: Remove
	dg-prune-output that doesn't match anything.
	* testsuite/23_containers/list/48101_neg.cc: Likewise.
	* testsuite/23_containers/multiset/48101_neg.cc: Likewise.
	* testsuite/23_containers/set/48101_neg.cc: Likewise.