[Native] Drop addChild from IrUtils2

We can reuse the same function from common `IrUtils`

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/IrUtils.kt

@@ -1066,12 +1066,15 @@ fun IrType.remapTypeParameters(
         else -> this
     }
 
-/* Copied from K/N */
 fun IrDeclarationContainer.addChild(declaration: IrDeclaration) {
     this.declarations += declaration
     declaration.setDeclarationsParent(this)
 }
 
+fun IrDeclarationContainer.addChildren(declarations: List<IrDeclaration>) {
+    declarations.forEach { this.addChild(it) }
+}
+
 fun <T : IrElement> T.setDeclarationsParent(parent: IrDeclarationParent): T {
     accept(SetDeclarationsParentVisitor, parent)
     return this

kotlin-native/backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/ir/util/IrUtils2.kt

@@ -97,15 +97,6 @@ inline fun <reified T> stub(name: String): T {
     } as T
 }
 
-fun IrDeclarationContainer.addChildren(declarations: List<IrDeclaration>) {
-    declarations.forEach { this.addChild(it) }
-}
-
-fun IrDeclarationContainer.addChild(declaration: IrDeclaration) {
-    this.declarations += declaration
-    declaration.accept(SetDeclarationsParentVisitor, this)
-}
-
 tailrec fun IrDeclaration.getContainingFile(): IrFile? {
     val parent = this.parent