This is going to be used by server-side flows. Note that the server-side
overload of `UpdateFromLegacy()` was not calling
`LegacyScoreDecoder.PopulateTotalScoreWithoutMods()`.
Computing the score without mods inline reduces reflection overheads
from constructing mod instances, which feels pretty important for
server-side flows.
There is one weird kink in the treatment of stable scores with score V2
active - they get the *legacy* multipliers unapplied for them because
that made the most sense. For all intents and purposes this matters
mostly for client-side replays with score V2. I'm not sure whether
scores with SV2 ever make it to submission in stable.
There may be minute differences in converted score due to rounding
shenanigans but I don't think it's worth doing a reverify for this.
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