But, when I ran the game, the boss bar was always there, even when I changed the setting in the config file.
I decided to see if something was wrong with my if statement in the entity onLivingUpdate, so I got rid of the else statement in the onLivingUpdate, and after that, it wouldn't display the boss bar at all.
Oh, I think the problem is that you're not actually referring to the the public static seeBossBar -- instead you created a second field boolean seeBossBar. You need to remove the word boolean from the line: boolean seeBossBar = config.get(Configuration.CATEGORY_GENERAL, "SeeBossBar", true).getBoolean(true);.
Also, just a suggestion on coding practice. In your onLivingUpdate() you don't need all the if else because you're testing a boolean that you're using. In other words, this entire section:
Also, in coding practice it is not necessary to compare the boolean to true or false. Instead of if(myBoolean == true) you should just put if(myBoolean). The comparison to true is redundant and actually can cause some subtle issues in some rare occasions.