How can I get rid of multiple “purchased on <my iPhone>” playlists?
Solution 1:
You can't consolidate them, but you can delete them. Just right click and click "Delete".
Solution 2:
To consolidate them into one play list, you need to go into the other otherplay lists, slect all the tracks, then you can right click and select "Add to playlist" and select one of the other "Purchased on iPhone..." playlists, repeat for all the extra playlists, then when they are consolidated delete the spare ones.
A similar process is detailed in this Apple KB.