Make `logins::import_multiple` not return a string

[bendk]

The current API has it returning a JSON-encoded string, but we never actually use it. We should try to change this code to not return anything. It seems that android-components and fenix don't actually look at the return and I don't think firefox-ios does either. I believe the only "consumer" of this value is our own wrapper code that used to use it to return metrics.

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