Removing old public keys
You can delete them with the second click options if you think so.

If you ever receive coins in that imported addresses you won't know if you don't have them imported in Electrum or in other client. Or if you've registered somewhere signing messages with it (like in #bitcoin-otc)

In case you want to keep them you will have to keep a backup of those keys separately by copying the wallet file as they won't become part of your Electrum seed.