Added a single source of SDL object IDs

This ensures that we don't accidentally interpret an ID from one system as an ID in another system.
Audio device IDs are not covered here, since they have a unique numbering system.

src/sensor/SDL_sensor_c.h

@@ -31,9 +31,6 @@ struct SDL_SensorDriver;
 
 /* Useful functions and variables from SDL_sensor.c */
 
-/* Function to get the next available sensor instance ID */
-extern SDL_SensorID SDL_GetNextSensorInstanceID(void);
-
 /* Initialization and shutdown functions */
 extern int SDL_InitSensors(void);
 extern void SDL_QuitSensors(void);