refactor(distinctUntilKeyChanged): remote thisArg

Author: kwonoj

src/operator/extended/distinctUntilKeyChanged.ts

@@ -1,10 +1,10 @@
 import {distinctUntilChanged} from '../distinctUntilChanged';
 
-export function distinctUntilKeyChanged<T>(key: string, compare?: (x: any, y: any) => boolean, thisArg?: any) {
+export function distinctUntilKeyChanged<T>(key: string, compare?: (x: any, y: any) => boolean) {
   return distinctUntilChanged.call(this, function(x, y) {
     if (compare) {
-      return compare.call(thisArg, x[key], y[key]);
+      return compare(x[key], y[key]);
     }
     return x[key] === y[key];
   });
-}
+}