[libc++] Revert OpenBSD-related changes to the documentation

This commit partially reverts 0954e2b2d038 and 3fa4cff97480, which
make changes to the libc++ documentation implifying that OpenBSD is
supported. Neither of these changes have been reviewed AFAICT, so
I'm reverting as a matter of enforcing:

1. That changes get reviewed before being committed
2. That we have a discussion and a support plan for supporting
   OpenBSD officially in libc++

Please note that I would be thrilled to support OpenBSD officially in
libc++, however doing so requires more than adding a note in the docs.
In particular, please make sure you read the note in [1] about setting
up CI testing for OpenBSD.

[1]: https://libcxx.llvm.org/#platform-and-compiler-support

Differential Revision: https://reviews.llvm.org/D109373

NOKEYCHECK=True
GitOrigin-RevId: ff7a332e6f5d4b09b4e437cc2c014367724b3eee
diff --git a/docs/UsingLibcxx.rst b/docs/UsingLibcxx.rst
index f37e23b..8631236 100644
--- a/docs/UsingLibcxx.rst
+++ b/docs/UsingLibcxx.rst
@@ -68,8 +68,8 @@
 
   $ clang++ -stdlib=libc++ test.cpp
 
-On systems where libc++ is the library in use by default such as macOS, FreeBSD
-and OpenBSD, this flag is not required.
+On systems where libc++ is the library in use by default such as macOS and FreeBSD,
+this flag is not required.
 
 
 .. _alternate libcxx:
diff --git a/docs/index.rst b/docs/index.rst
index 9fc66b0..a4bbac7 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -115,7 +115,6 @@
 =============== ========================= ============================
 macOS 10.9+     i386, x86_64, arm64       Building the shared library itself requires targetting macOS 10.11+
 FreeBSD 10+     i386, x86_64, arm
-OpenBSD         i386, x86_64, arm, arm64
 Linux           i386, x86_64, arm, arm64
 Windows         x86_64
 =============== ========================= ============================