chore: remove unnecessary Invalidator type

[Rich-Harris]

Via #12311 I noticed this is wrong — it doesn't take an argument, and by extension shouldn't take a type argument

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[dummdidumm]

@Conduitry do you remember why the private invalidator type ever optionally accepted a value? Looking at the corresponding store code it is indeed never called with a value.

[Rich-Harris]

I imagine it was just a typo