Patch servos code for move

- Have `Servo::attach` explicitly return -1 if it fails
- Check for -1 in `Servo::move` because `servoIndex` might be 0
- Make `attach` / `detach` calls conditional on `SERVO_LEVELING`
- Move `SERVO_LEVELING` define to `Conditionals.h`

Marlin/Marlin_main.cpp

@@ -36,8 +36,6 @@
   #endif
 #endif // ENABLE_AUTO_BED_LEVELING
 
-#define SERVO_LEVELING (defined(ENABLE_AUTO_BED_LEVELING) && defined(DEACTIVATE_SERVOS_AFTER_MOVE))
-
 #ifdef MESH_BED_LEVELING
   #include "mesh_bed_leveling.h"
 #endif