Remove leftover ImmutableArrayBuilder type

Author: Sergio0694

src/CommunityToolkit.Mvvm.SourceGenerators/Helpers/ImmutableArrayBuilder{T}.cs

@@ -205,18 +205,4 @@ private void ResizeBuffer(int sizeHint)
             ArrayPool<T?>.Shared.Return(oldArray, clearArray: typeof(T) != typeof(char));
         }
     }
-}
-
-/// <summary>
-/// Private helpers for the <see cref="ImmutableArrayBuilder{T}"/> type.
-/// </summary>
-file static class ImmutableArrayBuilder
-{
-    /// <summary>
-    /// Throws an <see cref="ArgumentOutOfRangeException"/> for <c>"index"</c>.
-    /// </summary>
-    public static void ThrowArgumentOutOfRangeExceptionForIndex()
-    {
-        throw new ArgumentOutOfRangeException("index");
-    }
 }