Outdated exposures info in Jetstream config docs

[mikewilli]

The end of this page notes that exposure metrics are not visualized in Experimenter, but this capability was added last year. We should remove this line from the docs: https://github.com/mozilla/experimenter-docs/blob/2df4fc46607ffa9bbd614401fbef91c1d4f362bd/docs/jetstream/configuration.md?plain=1#L458

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