compile this again

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6833 8aca7d54-2c30-db11-9de9-000461428c89
(eipi) code tidy: besch->is_available(year_month) removes duplicate code

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6834 8aca7d54-2c30-db11-9de9-000461428c89