Suits can be partially removed! The gloves can come off and the top of the suit unzipped, but effects really only come into place when the torso area of the suit is taken off. When this happens it becomes hard to breathe as the protective shielding the suit gives characters is removed. While it can still provide some, the longer Bull wears his suit like this the harder and harder it will be breathe. Especially since Kharas's atmosphere is toxic to most carbon-based lifeforms!

The maximum time he'll be able to go shirtless will be about 1-2 hours (depending on how robust he is) before there's just too little oxygen and he passes out.

Sits can be removed no problem in habitable environments! Unfortunately Kharas is not one of them.