doc: remove statement about client private keys

This statement was objectively false. Clients usually only need to
generate and/or own a private key if the server sends a
CertificateRequest during the TLS handshake, which is not a common case.

PR-URL: nodejs#41505
Reviewed-By: Filip Skokan <panva.ip@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>

doc/api/tls.md

@@ -17,7 +17,7 @@ const tls = require('tls');
 ## TLS/SSL concepts
 
 The TLS/SSL is a public/private key infrastructure (PKI). For most common
-cases, each client and server must have a _private key_.
+cases, each server must have a _private key_.
 
 Private keys can be generated in multiple ways. The example below illustrates
 use of the OpenSSL command-line interface to generate a 2048-bit RSA private