Remove erroneous Pin 12 definition

There is no switchable Power Supply.   Don't put it on Pin 12.   
(Pin 12 is actually FAN1)

Marlin/src/pins/pins_FORMBOT_TREX3.h

 // Use the RAMPS 1.4 Analog input 5 on the AUX2 connector
 #define FILWIDTH_PIN        5   // Analog Input
 
-#ifndef PS_ON_PIN
-  #define PS_ON_PIN        12
-#endif
-
 //
 // LCD / Controller
 //