[libc++] Remove the deprecated _LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDED macro (#73164)

We release-noted that we were going to remove it in the LLVM 18 timeline
and we've been issuing a warning since then.

Author: ldionne

libcxx/docs/ReleaseNotes/18.rst

@@ -87,16 +87,16 @@ Deprecations and Removals
   macro is provided to restore the previous behavior, and it will be supported in the LLVM 18 release only.
   In LLVM 19 and beyond, ``_LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT`` will not be honored anymore.
 
+- The ``_LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDED`` macro is not honored anymore in LLVM 18.
+  Please see the updated documentation about the hardening modes in libc++ and in particular the
+  ``_LIBCPP_VERBOSE_ABORT`` macro for details.
+
 Upcoming Deprecations and Removals
 ----------------------------------
 
 LLVM 18
 ~~~~~~~
 
-- The ``_LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDED`` macro will not be honored anymore in LLVM 18.
-  Please see the updated documentation about the hardening modes in libc++ and in particular the
-  ``_LIBCPP_VERBOSE_ABORT`` macro for details.
-
 - The headers ``<experimental/deque>``, ``<experimental/forward_list>``, ``<experimental/list>``,
   ``<experimental/map>``, ``<experimental/memory_resource>``, ``<experimental/regex>``, ``<experimental/set>``,
   ``<experimental/string>``, ``<experimental/unordered_map>``, ``<experimental/unordered_set>``,

libcxx/include/__verbose_abort

@@ -38,13 +38,6 @@ void __libcpp_verbose_abort(const char *__format, ...);
 // make sure that the program terminates but without taking any complex dependencies in this header.
 #if !defined(_LIBCPP_VERBOSE_ABORT)
 
-// Support _LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDED until LLVM 18, but tell people
-// to move to customizing _LIBCPP_VERBOSE_ABORT instead.
-#  if defined(_LIBCPP_AVAILABILITY_HAS_NO_VERBOSE_ABORT) && defined(_LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDED)
-#    undef _LIBCPP_AVAILABILITY_HAS_NO_VERBOSE_ABORT
-#    warning _LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDED is deprecated, please customize _LIBCPP_VERBOSE_ABORT instead
-#  endif
-
 #  if defined(_LIBCPP_AVAILABILITY_HAS_NO_VERBOSE_ABORT)
 // The decltype is there to suppress -Wunused warnings in this configuration.
 void __use(const char*, ...);