[libc++] Remove std::function in C++03

We've said that we'll remove `std::function` from C++03 in LLVM 16, so we might as well do it now before we forget.

Reviewed By: ldionne, #libc, Mordante

Spies: jloser, Mordante, libcxx-commits

Differential Revision: https://reviews.llvm.org/D135868

NOKEYCHECK=True
GitOrigin-RevId: dbe60caa922d5e4a30e4f335f33b2628dd82e3e7
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index 7befa46..8cc3d03 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -87,6 +87,9 @@
   the C++23 paper that makes these functions ``constexpr``, therefore the
   extension has been removed.
 
+- The ``_LIBCPP_ENABLE_CXX03_FUNCTION`` macro that allowed re-enabling the now-deprecated C++03 implementation of
+  ``std::function`` has been removed. Users who need to use ``std::function`` should switch to C++11 and above.
+
 Upcoming Deprecations and Removals
 ----------------------------------