The reason why you had to encrypt in the first place was because your corporate policy required it ... i.e., your IT department configured your MobileIron server to require encryption on your device, so once registered with MobileIron, you had to adopt the corporate policy.
I don't know of any Android devices that allow for decryption. That said, there's no good reason to decrypt. You are getting a security advantage with no real negatives. In theory there could be a micro-penalty in the battery consumption or i/o speed but in reality it's not going to be noticeable.
If you really can't stand the idea of encryption, you need to do a full factory reset. You'll lose all your data during that process so you'll want to backup first and restore once complete.