Fix Singlenozzle Standby issues (#21759)

Fixes #21758

Co-authored-by: Scott Lahteine <github@thinkyhead.com>

Marlin/src/inc/Conditionals_LCD.h

@@ -640,6 +640,16 @@
   #endif
 #endif
 
+/**
+ * Disable unused SINGLENOZZLE sub-options
+ */
+#if DISABLED(SINGLENOZZLE)
+  #undef SINGLENOZZLE_STANDBY_TEMP
+#endif
+#if !BOTH(HAS_FAN, SINGLENOZZLE)
+  #undef SINGLENOZZLE_STANDBY_FAN
+#endif
+
 /**
  * DISTINCT_E_FACTORS affects how some E factors are accessed
  */