build,doc: remove outdated `lint-md-build`

- In release guide
- In Travis config

Refs: https://github.com/nodejs/node/pull/20109

PR-URL: https://github.com/nodejs/node/pull/22991


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>