Without an index, maintaining an autoincrement column becomes too expensive, that's why MySQL requires an autoincrement column to be a leftmost part of an index.
You should remove the autoincrement property before dropping the key:
ALTER TABLE flight_class MODIFY id INT NOT NULL;
ALTER TABLE flight_class DROP PRIMARY KEY;
Note that you have a composite PRIMARY KEY which covers all three columns and id is not guaranteed to be unique.
If it happens to be unique, you can make it to be a PRIMARY KEY and AUTO_INCREMENT again:
ALTER TABLE flight_class MODIFY id INT NOT NULL PRIMARY KEY AUTO_INCREMENT;