doc: remove outdated s_client information in tls.md

There is a description of how to use s_client for testing of
renegotiation limits in the `tls` module documentation. The information
is somewhat out of scope, but it also may be somewhat problematic due to
changes/peculiarities (bugs?) in recent s_client. Remove the text.

Refs: https://github.com/nodejs/node/pull/25381#issuecomment-457067137

PR-URL: https://github.com/nodejs/node/pull/25678


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>