Remove warning that `serde` and `serde_derive` must be exactly equal

As pointed out by @Enselic in https://github.com/serde-rs/serde/issues/2584, this is not a direct concern as version mismatch checking was implemented already: https://github.com/serde-rs/serde/pull/2588

Created by  finchie  on June 4, 2025
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