build: remove `.txt` files from .gitignore

Refs: https://github.com/nodejs/node/pull/32670#issuecomment-609498003

PR-URL: https://github.com/nodejs/node/pull/32710


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>