Remove unused mjOption.apirate.

PiperOrigin-RevId: 815633571
Change-Id: Ib98f3eb2df47f96fa1fe6e987a3a0cd23a9479f6

unity/Runtime/Bindings/MjBindings.cs

@@ -5089,7 +5089,6 @@ public unsafe struct _mjVFS
 [StructLayout(LayoutKind.Sequential)]
 public unsafe struct mjOption_ {
   public double timestep;
-  public double apirate;
   public double impratio;
   public double tolerance;
   public double ls_tolerance;

unity/Runtime/Components/MjGlobalSettings.cs

@@ -156,7 +156,6 @@ public struct MjSizeStruct {
 public struct MjOptionStruct {
 
   // "timestep" and "gravity" come from global settings.
-  // "apirate" is only relevant to HAPTIX.
   [Tooltip("Ratio of frictional-to-normal constraint impedance.")]
   public float ImpRatio;
   [Tooltip("Global magnetic flux used by magnetometer sensors.")]