[libc++] Remove _LIBCPP_DISABLE_AVAILABILITY macro

This was slated for removal years ago, so now's a good time to remove it.

Author: ldionne

libcxx/docs/ReleaseNotes/20.rst

@@ -85,6 +85,9 @@ Deprecations and Removals
 - The function ``__libcpp_verbose_abort()`` is now ``noexcept``, to match ``std::terminate()``. (The combination of
   ``noexcept`` and ``[[noreturn]]`` has special significance for function effects analysis.)
 
+- The ``_LIBCPP_DISABLE_AVAILABILITY`` macro that was used to force-disable availability markup has now been removed.
+  Whether availability markup is used by the library is now solely controlled at configuration-time.
+
 Upcoming Deprecations and Removals
 ----------------------------------
 

libcxx/include/__configuration/availability.h

@@ -67,14 +67,6 @@
 //
 // [1]: https://clang.llvm.org/docs/AttributeReference.html#availability
 
-// For backwards compatibility, allow users to define _LIBCPP_DISABLE_AVAILABILITY
-// for a while.
-#if defined(_LIBCPP_DISABLE_AVAILABILITY)
-#  if !defined(_LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS)
-#    define _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS
-#  endif
-#endif
-
 // Availability markup is disabled when building the library, or when a non-Clang
 // compiler is used because only Clang supports the necessary attributes.
 #if defined(_LIBCPP_BUILDING_LIBRARY) || defined(_LIBCXXABI_BUILDING_LIBRARY) || !defined(_LIBCPP_COMPILER_CLANG_BASED)