[libc++] Fix feature test macro for __cpp_lib_to_chars

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.

Differential Revision: https://reviews.llvm.org/D114243

NOKEYCHECK=True
GitOrigin-RevId: e0f58444e126e80a0d43eb5b88622799910195e6
diff --git a/include/version b/include/version
index d2286f5..7c16ac8 100644
--- a/include/version
+++ b/include/version
@@ -149,7 +149,7 @@
 __cpp_lib_three_way_comparison                          201907L <compare>
 __cpp_lib_to_address                                    201711L <memory>
 __cpp_lib_to_array                                      201907L <array>
-__cpp_lib_to_chars                                      201611L <utility>
+__cpp_lib_to_chars                                      201611L <charconv>
 __cpp_lib_to_underlying                                 202102L <utility>
 __cpp_lib_transformation_trait_aliases                  201304L <type_traits>
 __cpp_lib_transparent_operators                         201510L <functional> <memory>