Disallow extension properties with backing fields

#KT-1682 Fixed

compiler/testData/codegen/box/labels/propertyAccessorFunctionLiteral.kt

@@ -5,19 +5,9 @@ val Int.getter: Int
         }.invoke()
     }
 
-var Int.setter: Int = 1
-    set(i: Int) {
-        $setter = {
-            this@setter
-        }.invoke()
-    }
-
 fun box(): String {
     val i = 1
     if (i.getter != 1) return "getter failed"
 
-    i.setter = 2
-    if (i.setter != 1) return "setter failed"
-
     return "OK"
-}
+}