ssi-jwt: remove unused serde_with dependency

[burdettadam]

The ssi-jwt crate declares serde_with 2.3.2 but does not use it in production, tests, examples, or generated code. Removing the unused declaration eliminates GHSA-7gcf-g7xr-8hxj from downstream dependency graphs without changing the public API or enabled features.\n\nValidated with cargo check -p ssi-jwt and cargo tree -p ssi-jwt.

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