Bought an iPad but it's supervised.Is there a way to remove it?
Supervised devices lock in management, so your only recourse is to wipe the device. At that point you may discover that it’s also an enrolled device (DEP) and wiping does not unenroll from DEP as that is different than a MDM profile. 

You cannot, by yourself, remove the iPad from DEP. The iPad has all these restrictions because it was previously managed by a company or school. You’ll need to figure out which school or company it is and send their IT and email and hope that they will remove it from their program for you. Their and IT and only their IT can unenroll an iPad from DEP.

You likely have bought an iPad that a student/teacher/employee/malicious actor stole. I doubt that an IT department knowledgeable enough to use DEP wouldn’t know to disown the devices before resale.

Also, I know this question will likely attract a lot of attention by students wishing to remove their devices from DEP: short answer, you can’t. Long answer: if you jailbreak the device you can remove the profile. This change will persist while you are on that iOS version and maybe once you update however if you restore the device it’ll come back. I of course don’t recommend this and you’ll violate your school policy (and IT will know, it’s not hard to detect a jailbroken device).