build: use do_not_edit variable where possible

We should not use hardcoded string to warn users about
file was generated by configure script. Since we already
have do_not_edit variable we can use it

PR-URL: https://github.com/nodejs/node/pull/12610


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>