Boot your phone to TWRP recovery mode by turn off your phone then presses Vol up + Power to boot into bootloader then choose RECOVERY. There is a more simple way, by using “adb reboot recovery”.
Once came into TWRP main menu, now tap Mount -> Mount system then taps Home.
Tap Advanced -> File Manager, browse to “/system/xbin” folder. From there delete su, daemonsu or busybox if it exists. Also browse to “/system/app” the get rid of Superuser.apk.