I want to remove activation lock

The only way to remove activation lock when you have lost the info is contacting Apple with the original (not a copy) receipt showing the serial number. This is not always possible, but is the only way to remotely have a chance.

You can't.

Activation Lock is a theft deterrent feature with no bypass or workaround.

Without the owner's authorisation (Apple ID & password), no thief or anyone including you can re-use the device.