[libc++] Remove _LIBCPP_DISABLE_AVAILABILITY macro (llvm#112952)

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

Author: ldionne

libcxx/docs/ReleaseNotes/20.rst

@@ -102,6 +102,9 @@ Deprecations and Removals
   headers as an extension and only deprecates them. The ``_LIBCPP_DISABLE_DEPRECATION_WARNINGS`` macro can be defined to
   suppress deprecation for these headers.
 
+- 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

@@ -67,13 +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)
-#  undef _LIBCPP_HAS_VENDOR_AVAILABILITY_ANNOTATIONS
-#  define _LIBCPP_HAS_VENDOR_AVAILABILITY_ANNOTATIONS 0
-#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)