Drop BigInteger in favour of integer
See: http://www.tryton.org/~irclog/2017-09-28.log.html#t2017-09-28%2012:45
All legacy (hg-git based) Projects have been automatically migrated to native with the upgrade of 2025-01-23
See: http://www.tryton.org/~irclog/2017-09-28.log.html#t2017-09-28%2012:45