tls: make deprecated tls.createSecurePair() use public API

Make the deprecated `tls.createSecurePair()` method use other public
APIs only (`TLSSocket` in particular).

Since `tls.createSecurePair()` has been runtime-deprecated only
since Node 8, it probably isn’t quite time to remove it yet,
but this patch removes almost all of the code complexity that
is retained by it.

The API, as it is documented, is retained. However, it is very likely
that some users have come to rely on parts of undocumented API
of the `SecurePair` class, especially since some of the existing
tests checked for those. Therefore, this should definitely be
considered a breaking change.

PR-URL: https://github.com/nodejs/node/pull/17882


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>