tls: don't shadow the tls global with a local

`tls` shadows the global `tls` require, and isn't indicative of the
arument type.

PR-URL: https://github.com/nodejs/node/pull/25861


Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>