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

This commit partially reverts 0954e2b2 and 3fa4cff9, 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