Revert "[Docs] Add release note about Clang-defined target OS macros (#79879)"

This reverts commit b40d5b1.

The target OS macros work is included in the 18.x release. Move the
release note to the release branch.

Author: zixu-w

clang/docs/ReleaseNotes.rst

@@ -49,24 +49,6 @@ ABI Changes in This Version
 AST Dumping Potentially Breaking Changes
 ----------------------------------------
 
-Clang Frontend Potentially Breaking Changes
--------------------------------------------
-
-Target OS macros extension
-^^^^^^^^^^^^^^^^^^^^^^^^^^
-A new Clang extension (see :ref:`here <target_os_detail>`) is enabled for
-Darwin (Apple platform) targets. Clang now defines ``TARGET_OS_*`` macros for
-these targets, which could break existing code bases with improper checks for
-the ``TARGET_OS_`` macros. For example, existing checks might fail to include
-the ``TargetConditionals.h`` header from Apple SDKs and therefore leaving the
-macros undefined and guarded code unexercised.
-
-Affected code should be checked to see if it's still intended for the specific
-target and fixed accordingly.
-
-The extension can be turned off by the option ``-fno-define-target-os-macros``
-as a workaround.
-
 What's New in Clang |release|?
 ==============================
 Some of the major new features and improvements to Clang are listed
@@ -115,17 +97,6 @@ Non-comprehensive list of changes in this release
 New Compiler Flags
 ------------------
 
-.. _target_os_detail:
-
-Target OS macros extension
-^^^^^^^^^^^^^^^^^^^^^^^^^^
-A pair of new flags ``-fdefine-target-os-macros`` and
-``-fno-define-target-os-macros`` has been added to Clang to enable/disable the
-extension to provide built-in definitions of a list of ``TARGET_OS_*`` macros
-based on the target triple.
-
-The extension is enabled by default for Darwin (Apple platform) targets.
-
 Deprecated Compiler Flags
 -------------------------