Remove rhythm change bonus from taiko SR

[chayleaf]

All the current meta farm maps abuse rhythm changes. The maps with hard 1/6 patterns exist (and DT'ing the 1/6 farm maps is only doable for like 10 people), but the difficulty doesn't come from rhythm changes themselves. Something needs to be done to the maps like that (at least acc pp needs to account for this kind of stuff, but also for OD), but in the meanwhile, I propose a simple rhythm change bonus removal.

[cory2067]

Here's results from a bit of testing.

pavlukivan (rank 500): https://paste.ee/p/LHKfW
Cychloryn (rank 2500): https://paste.ee/p/FX8DR
_yu68 (rank 1): https://paste.ee/p/9FXqh
applerss (rank 8): https://paste.ee/p/Zdiqv

Unsurprisingly, 1/4+1/6 patterns are nerfed a lot, 1/8 doublets are nerfed A LOT, and rhythms with only 1/2 and 1/4 rhythms are basically unaffected.

I actually think the nerf is a bit too aggressive. Aether Ritual [Ritual Oni] is currently 8.06 stars, but this change brings it to 4.49 stars. If we took Ritual Oni and replaced the doublets with a long 1/4 stream, it would actually be 4.55 stars, harder than the original.

Maybe this is debatable, but I personally think 1/8 doublets are inherently harder than an even 1/4 stream (but not too much harder).

This change also exacerbate these other issues:

• Color changes like kddkddk/dkkdkkd being overweight: _yu68's top play would become Pochiko no Shiawase na Nichijou, which uses these patterns a lot
• pp calculations being overweight in the beginning, underweight at high level play. With this change, applerss' 100th top play is under 300pp. Meanwhile, Muzukashii diffs still can be well over 100pp.

Maybe instead of removing rhythm bonus, a better solution might be to reduce it. (or just come up with a different way of calculating the rhythm bonus)

-- Cychloryn

[chayleaf]

We need a different way to calculate pp for 1/6, because some of them are hard.

Color changes are overweighted, but not as much, and fixing them is too hard for a quick patch.

Ritual is doable for futsuu players, because it's playable like finishers. Imo it's fine to allow small fluctuations both up and down compared to 1/4 streams.

[chayleaf]

High difficulties being underweighted is related to acc pp. Imo it's too early to do anything about it (1/8 doubles are hard to acc, for example), but for a quick fix we can just use some pp bonus for high difficulty.

[cory2067]

btw a note on how to reproduce:

Apply the changes from this PR first to get (almost) 1-1 with stable
ppy/osu#5318

In strain.cs, change this constant from 1 to something smaller

if (hasRhythmChange(current))
                    addition += 1;