Let's revert the changes for now, as it cannot be included in the 2.2.0
release.

My comment on #257:

> A blocker is OpenSSL::SSL::SSLContext#add_certificate_chain_file. It
> has a pending change and I don't want to include it in an incomplete
> state.
>
> The initial implementation in commit 46e4bdba40c5 was not really
> useful. The issue is described in #305. #309 extended it
> to take the corresponding private key together. However, the new
> implementation was incompatible on Windows and was reverted by #320 to
> the initial one.
>
> (The prerequisite to implement it in) an alternative way is #288, and
> it's still cooking.

This effectively reverts the following commits:

 - dacd08937ccd ("ssl: suppress test failure with SSLContext#add_certificate_chain_file", 2020-03-09)
 - 46e4bdba40c5 ("Add support for SSL_CTX_use_certificate_chain_file. Fixes #254.", 2019-06-13)

https://github.com/ruby/openssl/commit/ea925619a9