Drop obsolete string

Marlin/src/core/language.h

 #define MSG_FILAMENT_CHANGE_INSERT_LCD      "Insert filament and press button"
 #define MSG_FILAMENT_CHANGE_INSERT_M108     "Insert filament and send M108"
 
-#define MSG_SERIAL_ERROR_MENU_STRUCTURE     "Error in menu structure"
-
 #define MSG_ERR_EEPROM_WRITE                "Error writing to EEPROM!"
 
 #define MSG_STOP_BLTOUCH                    "STOP called because of BLTouch error - restart with M999"