You are going to enjoy this cool feature!!!
The #define for SERVO_ENSTOPS is always defined in the Configuration.h file. I suspect that early in the development of the Auto Bed Leveling it was conditional. But that is not the case now. I think this comment is causing you concern when you can just ignore it.
Just delete the // in front of:
// #define PROBE_SERVO_DEACTIVATION_DELAY 300
And the servo should automatically quit being powered. But I kicked up the time a bit. 1/3 of a second seemed a little bit light. I have mine at 1/2 second or a value of 500.