Undef unused 2nd serial option(s) (#21331)

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

Marlin/src/inc/Conditionals_post.h

   #define HAS_MULTI_SERIAL 1
 #endif
 
-#if ENABLED(MEATPACK_ON_SERIAL_PORT_1) || BOTH(HAS_MULTI_SERIAL, MEATPACK_ON_SERIAL_PORT_2)
+#if !HAS_MULTI_SERIAL
+  #undef MEATPACK_ON_SERIAL_PORT_2
+#endif
+#if EITHER(MEATPACK_ON_SERIAL_PORT_1, MEATPACK_ON_SERIAL_PORT_2)
   #define HAS_MEATPACK 1
 #endif