Remove unnecessary cloning (#4263)
Seems like Externalities already [return a vector](https://github.com/paritytech/polkadot-sdk/blob/ffbce2a8 /substrate/primitives/externalities/src/lib.rs#L86), so calling `to_vec` on a vector just results in an unneeded copying. Co-authored-by: Liam Aharon <[email protected]>