Drop intermediate float in HEATER_0_USES_MAX6675 test

Marlin/temperature.cpp

   updateTemperaturesFromRawValues(); // also resets the watchdog
 
   #if ENABLED(HEATER_0_USES_MAX6675)
-    float ct = current_temperature[0];
-    if (ct > min(HEATER_0_MAXTEMP, 1023)) max_temp_error(0);
-    if (ct < max(HEATER_0_MINTEMP, 0.01)) min_temp_error(0);
+    if (current_temperature[0] > min(HEATER_0_MAXTEMP, 1023)) max_temp_error(0);
+    if (current_temperature[0] < max(HEATER_0_MINTEMP, 0.01)) min_temp_error(0);
   #endif
 
   #if (ENABLED(THERMAL_PROTECTION_HOTENDS) && WATCH_TEMP_PERIOD > 0) || (ENABLED(THERMAL_PROTECTION_BED) && WATCH_BED_TEMP_PERIOD > 0) || DISABLED(PIDTEMPBED) || HAS_AUTO_FAN