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: nodejs/node#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>

Author: apapirovski

lib/tls.js

@@ -44,8 +44,6 @@ const _tls_wrap = require('_tls_wrap');
 exports.CLIENT_RENEG_LIMIT = 3;
 exports.CLIENT_RENEG_WINDOW = 600;
 
-exports.SLAB_BUFFER_SIZE = 10 * 1024 * 1024;
-
 exports.DEFAULT_CIPHERS =
     process.binding('constants').crypto.defaultCipherList;