[libc++] Remove the old HTML documentation

This commit finishes moving the <atomic> design documents to the RST
documentation and removes the old documentation. https://libcxx.llvm.org
is already pointing to the new documentation only now, so the removal of
the old documentation is really a NFC.

I went over the old documentation and I don't think we're leaving anything
important behind - I think everything important was mentionned in the RST
documentation anyway.