Remove mentions of None from docs

Author: tpoomlmly

programming/sr/cheat_sheet.md

@@ -138,13 +138,6 @@ R.servo_board.servos[1].position = 1
 
 You can also set the position to `0`, which is the approximate centre.
 
-This is different to setting the position to `None`, which will unpower the servo.
-
-~~~~~ python
-# This servo is now unpowered, and will move more freely.
-R.servo_board.servos[11].position = None
-~~~~~
-
 ## Camera
 
 ### Taking a photo

programming/sr/servos/index.md

@@ -69,17 +69,6 @@ While it is possible to retrieve the last position a servo was set to,
 this does not guarantee that the servo is currently in that position.
 </div>
 
-Turning off a servo
--------------------
-
-You can also set a servo output to be unpowered, so that there is no longer a PWM signal from the output on the servo board.
-
-You can unpower the servo output by setting the position to `None`.
-
-~~~~~ python
-r.servo_board.servos[11].position = None
-~~~~~
-
 [How the set position relates to the servo angle](#ServoAngle) {#ServoAngle}
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