tls: remove SLAB_BUFFER_SIZE

This constant has not been in use for many years now and the test
alongside it is invalid, as well as flaky.

PR-URL: https://github.com/nodejs/node/pull/21199


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Ujjwal Sharma <usharma1998@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>