Remove ISample.Length

[peppy]

This has not been populated for a while and is not used osu! side. Rather than having an incorrectly zero value, let's remove it for now.

[frenzibyte]

There's technically one indirect usage of this osu!-side: https://github.com/ppy/osu/blob/ad2cd0ba8fb0b640155a704d2f39069eabec4985/osu.Game/Rulesets/Objects/Drawables/DrawableHitObject.cs#L488-L489

As far as I understand, Length never mattered because the transform end time usually exceeds the length of the hit sample. I also thought the death of a DHO does not interrupt any played sample, deeming the above basically pointless, but turns out it does currently (see here). It's probably more ideal to remove that workaround and/or limit it to looping samples, since it would be a waste to prepare a new DHO just because the previous one is busy waiting for a sample to finish playing (especially since samples already have their own pool).

This is getting beyond the scope of this PR but I think it's worth mentioning anyway since we're removing ISample.Length now and the above intended to rely on it.

[smoogipoo]

The above sounds very relevant, actually.