Removing iCloud from iPhone 6
How can I remove iCloud from my phone with a laptop
Is it locked to your own iCloud account? If so, you'll need to log into iCloud.com and remove it from Find My Device.
If it is not your iCloud account, you would need original proof of purchase to submit to Apple to prove ownership of the phone so they can remove the lock.
Based on my understanding, any other iCloud lock removal method done via a computer is not permanent and the lock will come back with software update.