Remove ImportedModule.id field?

[bendk]

According to clippy this is dead code. Should we remove it if it's not being used?

https://github.com/mozilla/application-services/blob/b82517cde306274216874d6e6eec9f4042cdf9c6/components/support/nimbus-fml/src/intermediate_representation.rs#L722-L728

@jhugman (I moved this question out of the #5128 thread since I didn't want it to block the merge).

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