fix redefinition errors for redundant_temperature

Removing redundant_temperature from public section and leaving it in the
private section

Marlin/temperature.cpp

   int16_t Temperature::target_temperature_bed = 0;
 #endif
 
-#if ENABLED(TEMP_SENSOR_1_AS_REDUNDANT)
-  float Temperature::redundant_temperature = 0.0;
-#endif
-
 #if ENABLED(PIDTEMP)
   #if ENABLED(PID_PARAMS_PER_HOTEND) && HOTENDS > 1
     float Temperature::Kp[HOTENDS] = ARRAY_BY_HOTENDS1(DEFAULT_Kp),

Marlin/temperature.h

 
     static volatile bool in_temp_isr;
 
-    #if ENABLED(TEMP_SENSOR_1_AS_REDUNDANT)
-      static float redundant_temperature;
-    #endif
-
     static uint8_t soft_pwm_amount[HOTENDS],
                    soft_pwm_amount_bed;