I see #898546: Clean up the Filter module upgrade path, which deals with some more in-depth issues. Filing this separately because this is small and simple (easy win) and that issue is over a year old.
Basically, during the upgrade in filter_update_7003(), the D6 filters table is renamed d6_upgrade_filter to migrate data. After the upgrade, we're left with an unused table. We should drop it, as it stems from a core Drupal 6 table.
Patch attached.