Double tap to unlock is a feature of Talkback, so if your will disable Talkback, double tap to unlock will also be disabled. To disable double tap to unlock, please do the following.
1. open Settings.
2. Select Accessibility.
3. Under Accessibility services, uncheck Talkback.
4. That`s it. After disabling Talkback, you won`t have to double tap your phone anymore to unloc