These APIs/configs are not approved by the Ruby core, so they can't be
released to the public. This means having them in the codebase will
block other fixes/features from being released as well.

So this commit removes those exposed interfaces to unblock the release.

Hopefully when https://bugs.ruby-lang.org/issues/18996 is approved we
can re-implement better APIs.

https://github.com/ruby/reline/commit/f7a961c550