I don't know about remove it outright, but if you're able to change the variables in the browser (SugarCube.State.active.variables), you can set fitness to some large numbers so that even with decay, that you may never see the bar go down again.Is there some way to remove the fitness decay?
For example, 100 is generally the fitness cap attainable regularly in-game from working out. i don't know if there's a coded hardcap, but if you set your fitness variable to 9999, you could play the game for like a year in-game until you'd start to see the fitness bar go down.