libstdc++: Remove c++20-specific dg-error directives in test

libstdc++: Remove c++20-specific dg-error directives in test

I added extra dg-error directives for C++20 to match the extra errors
caused by some of the call stack being constexpr in C++20. Since Jason's
changes to reduce those errors, those dg-error lines no longer match.
This removes them again.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* testsuite/20_util/specialized_algorithms/memory_management_tools/destroy_neg.cc:
	Remove dg-error lines for C++20-only errors.