Allow redundant temp sensor with SINGLENOZZLE (#15650)

Marlin/src/inc/Conditionals_LCD.h

 #if EITHER(SINGLENOZZLE, MIXING_EXTRUDER)         // One hotend, one thermistor, no XY offset
   #undef HOTENDS
   #define HOTENDS       1
-  #undef TEMP_SENSOR_1_AS_REDUNDANT
   #undef HOTEND_OFFSET_X
   #undef HOTEND_OFFSET_Y
 #endif

Marlin/src/inc/SanityCheck.h

     #error "Marlin supports a maximum of 6 EXTRUDERS."
   #endif
 
-  #if ENABLED(TEMP_SENSOR_1_AS_REDUNDANT)
-    #error "EXTRUDERS must be 1 with TEMP_SENSOR_1_AS_REDUNDANT."
-  #endif
-
   #if ENABLED(HEATERS_PARALLEL)
     #error "EXTRUDERS must be 1 with HEATERS_PARALLEL."
   #endif
     #error "TEMP_SENSOR_1 is required with 2 or more HOTENDS."
   #elif !ANY_PIN(TEMP_1, MAX6675_SS2)
     #error "TEMP_1_PIN not defined for this board."
+  #elif ENABLED(TEMP_SENSOR_1_AS_REDUNDANT)
+    #error "HOTENDS must be 1 with TEMP_SENSOR_1_AS_REDUNDANT."
   #endif
   #if HOTENDS > 2
     #if TEMP_SENSOR_2 == 0