UDL can't have a `Remote` enums with fields

[mhammond]

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Eg, a simple enum: https://github.com/mozilla/uniffi-rs/blob/4cbb8f10a1e8171d0b36114f126cb0abeea0f59a/fixtures/ext-types/lib/src/ext-types-lib.udl#L71-L75

If we had an enum with fields, we'd need an [Enum, Remote] interface {... }, but that would fail - https://github.com/mozilla/uniffi-rs/blob/aa200a689aa2fca77f8d4b73051c98cdeaecce3c/uniffi_udl/src/attributes.rs#L458

via #2334