how can i delete linked in icloud (i forgotten the iclould)

If you've forgotten your password : http://iforgot.apple.com

If it's on the activation screen and you've forgotten your id then do you have any of the account's downloads in your computer's iTunes library : Recovering a forgotten iTunes Store account name - Apple Support

Or try finding it via http://appleid.apple.com : How to find your Apple ID - Apple Support

If you still can't find your id, and if you were the original purchaser of the iPad (i.e. it wasn't bought used), then you could try going to an Apple Store (make a Genius Bar appointment if you do) or try contacting iTunes Support online (http://www.apple.com/support/itunes/contact/) and see if they can/will remove the iPad from the account - they are likely to want to see proof of ownership e.g. original receipt with the serial number on it