Override/Implement members: don't add 'external' modifier

#KT-33466 Fixed

idea/testData/codeInsight/overrideImplement/overrideExternalFunction.kt

@@ -0,0 +1,7 @@
+open class A {
+    open external fun foo()
+}
+
+class B : A() {
+    <caret>
+}

idea/testData/codeInsight/overrideImplement/overrideExternalFunction.kt.after

@@ -0,0 +1,9 @@
+open class A {
+    open external fun foo()
+}
+
+class B : A() {
+    override fun foo() {
+        super.foo()
+    }
+}