Do not expose TXTNODE_MAX and cleanup related code/comments

Change-Id: Ia114688ebbdf81122b81183456924c6b9cff835d
Reviewed-on: https://gerrit.libreoffice.org/7105
Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>
Reviewed-by: Matteo Casalin <matteo.casalin@yahoo.com>
Tested-by: Matteo Casalin <matteo.casalin@yahoo.com>