This is yet another small piece of clean-up of the `FontLoader`-code, since we've not used this `id`-property for anything ever since PR 6571 (which landed almost seven years ago). Furthermore, by default we're also not even using that code-path now since the Font Loading API will always be used when available.
*Please note:* This is tagged `[api-minor]` since it's technically observable from the outside, however no user ought to be directly interacting with these CSS font rules.
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