misc: move predictive-perf off renderer i18n

[brendankenny]

Due to the kind of weird legacy of the renderer's i18n.js (coming from a util shared between the report and core), predictive-perf ended up using the renderer i18n class to do a simple milliseconds format rather than using the i18n pipeline (albeit for a string that no human will see or probably ever needs).

Now, whether or not predictive-perf needs a properly localized and formatted milliseconds (instead of just doing rounding and adding ms itself locally) is another question, but this is basically free, so I don't think it matters. Either way it allows the renderer i18n to not have optional strings, which was my original goal with the change, so happy to change if someone feels strongly :)