...as well as 'bitrate' you can also choose 'Kind' but the effect is still the same. Sorting by 'Kind' will sort all the AAC and Apple Lossless files into separate groups. Highlight the AAC files and hit delete. First you will be asked if you want to delete them from your library (yes) then you will be asked if you want to delete the files themselves. As Overdose said, if you delete them they will be put in the trash where you can move them elsewhere.