Override/Implement: Drop 'impl' modifier for non-impl class members

#KT-18469 Fixed

idea/testData/multiModuleQuickFix/implementMembersInImplClassNonImplInheritor/header/header.kt

@@ -0,0 +1,3 @@
+header abstract class Bar {
+    abstract fun foo()
+}

idea/testData/multiModuleQuickFix/implementMembersInImplClassNonImplInheritor/header/header.kt.after

@@ -0,0 +1,3 @@
+header abstract class Bar {
+    abstract fun foo()
+}