If you have access to the polkadot-js/apps UI to your chain, you can get the keys listed in the Developer -> Chain state section. Select the entry you want as well as the params and it will also show the key for that entry. Simply copy it and you should be good to go.
An example for
futureCodeHash(2000) on Rococo would be
Next up you can use this value in the Developer -> Extrinsics section to remove the storage via the
It has been deployed for this type of cleanup by the Council on Kusama/Polkadot a couple of times in the past (mostly to remove old state that has moved to different locations).
As a first round, I would always suggest that you test the process against a dev chain first, unless it is something disposable.