Disallow extension properties with backing fields

#KT-1682 Fixed

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

@@ -3,27 +3,9 @@ val Int.getter: Int
         return this@getter
     }
 
-var Int.setter1: Int = 2
-    get() {
-        return this@setter1
-    }
-    set(i: Int) {
-        $setter1 = this@setter1
-    }
-
-var Int.setter2: Int = 2
-    set(i: Int) {
-        $setter2 = this@setter2
-    }
-
 fun box(): String {
     val i = 1
     if (i.getter != 1) return "getter failed"
 
-    i.setter1 = 2
-    if (i.setter1 != 1) return "setter1 failed"
-    i.setter2 = 2
-    if (i.setter2 != 1) return "setter2 failed"
-
     return "OK"
-}
+}