Improved dropping article_flag indexes.

Fixed bug#9280 - Database upgrade procedure problems when upgrading
database to 3.2 that has been upgraded from 2.4 previously.
http://bugs.otrs.org/show_bug.cgi?id=9280

Databases that have been created on OTRS 2.4 and then upgraded don't
have these indexes, if they have been created on OTRS 3.0 and then
upgraded they do have these.