We would have been defining it in <utility> instead of <charconv>. For
the time being, this doesn't change anything since we don't implement
the feature test macro anyways.
Also, as a fly-by, this removes obsolete feature test macro tests. There
was a brief time back in the days when we wrote feature test macro tests
manually. In particular, we had test files for cpp_lib_to_chars and
cpp_lib_memory_resource. Since we now have a principled way of generating
these tests with scripts, this commit removes the obsolete (and empty)
tests for these two feature test macros.