Error: "Operation not permitted", when attempting to remove su binary placed by KingoRoot

I rooted my P8-Lite ALE-L21 (Android 5.0.1) with supersu.zip through TWRP recovery. After that I installed KingoRoot and KingoRoot replaced *su* binary with some other *su* binary which is obsolete for my Android version and not working. So I modified supersu.zip and placed *su* binary into another directory in /data/local. Now my working *su* binary is in /data/local and it works fine with adb, but does not work with SuperSU and other applications which use root privilege. To solve that issue, I tried to remove these old *su* binaries:/system/bin/su and /system/xbin/su, with the following commands unsuccessfully:

$ adb shell
shell@hwALE-H:/ $ /data/local/su
root@hwALE-H:/ # mount -o remount,rw /system
root@hwALE-H:/ # rm /system/xbin/su
rm: /system/xbin/su: Operation not permitted
1|root@hwALE-H:/ # rm -rf /system/xbin/su
rm: /system/xbin/su: Operation not permitted

* What is this error: *Operation not permitted*? * How can I remove those su files?