Adjust rounding in mod select difficulty multiplier to match song select footer

The 0.01 `Precision` spec on `DifficultyMultiplierDisplay.Current` would
cause the difficulty multiplier to use a different midpoint rounding
strategy than `double.ToString()`, which is the one that the song select
footer relies on. For example, a value of 0.015 would be rounded down
to 0.01 by `double.ToString()`, but rounded up to 0.02
by `BindableDouble`.

Fix the discrepancy by just deleting the `Precision` spec. Since the
value of the bindable would go through `ToLocalisableString(@"N2")`
anyway, it was redundant as is.

Fixes #19889.

osu.Game/Overlays/Mods/DifficultyMultiplierDisplay.cs

@@ -31,10 +31,7 @@ namespace osu.Game.Overlays.Mods
             set => current.Current = value;
         }
 
-        private readonly BindableNumberWithCurrent<double> current = new BindableNumberWithCurrent<double>(1)
-        {
-            Precision = 0.01
-        };
+        private readonly BindableNumberWithCurrent<double> current = new BindableNumberWithCurrent<double>(1);
 
         private readonly Box underlayBackground;
         private readonly Box contentBackground;