Generating remove() for Iterator using same mechanism.

compiler/testData/codegen/box/builtinStubMethods/Iterator.kt

@@ -0,0 +1,13 @@
+class MyIterator<T>(val v: T): Iterator<T> {
+    override fun next(): T = v
+    override fun hasNext(): Boolean = true
+}
+
+fun box(): String {
+    try {
+        (MyIterator<String>("") as MutableIterator<String>).remove()
+        throw AssertionError()
+    } catch (e: UnsupportedOperationException) {
+        return "OK"
+    }
+}