Remove poorly-fitted "legacy combo counter" abstraction

[bdach]

• Closes combo counter & longest combo counter miscount #29727
• Supersedes / closes Fix legacy combo counter sometimes miscount #29811

This is probably going to be hell to review, and I apologise for this, but I believe it is best to course correct now before this gets worse.

The long and short of it is that #26254, namely probably somewhere around 78cb6b6, regressed the "default legacy combo counter" display logic. When reviewing the code for #29811 and attempting to determine why, I found the code quality of the counters frankly shocking, with stuff like this:

osu/osu.Game/Skinning/LegacyDefaultComboCounter.cs

Lines 54 to 61 in fd887b2

	this.Delay(big_pop_out_duration - 140).Schedule(() =>
	{
	base.OnCountIncrement();
	DisplayedCountText.ScaleTo(1).Then()
	.ScaleTo(1.1f, small_pop_out_duration / 2, Easing.In).Then()
	.ScaleTo(1, small_pop_out_duration / 2, Easing.Out);
	}, out scheduledPopOut);

osu/osu.Game/Skinning/LegacyComboCounter.cs

Lines 137 to 143 in fd887b2

	protected virtual void OnCountIncrement()
	{
	if (DisplayedCount < Current.Value - 1)
	DisplayedCount++;
	DisplayedCount++;
	}

This, dear reader, is a scheduled base call which mutates internal state in a manner that seems inconsistent with previous code and frankly arbitrary, which to me is NOPE NOPE NOPE territory. I want stuff like that gone from the codebase as soon as possible because it is not sane and we have lived several examples of this already.

To that end, this PR reverts LegacyDefaultComboCounter to what it was at e7b4a2f, fixing the issue, and moves enough of the machinery that used to live in the abstract eldritch entity into LegacyManiaComboCounter so that it can function standalone.

Had it not been for this review thread I probably would have searched for a more moderate resolution, but this being brought up already at review time definitely has me wanting to nip this in the bud before the abstract entity grows to inscrutability.

[peppy]

In hindsight, trying to salvage that PR rather than fix clear mistakes was a bad decision.