How to delete all emails from inbox in iPad 8.1.3?
Since you can't perform this trick... if you're that desperate, you can update to iOS 9 Public Beta and this feature will become present.

Alternatively, you can jailbreak your iPad and get a tweak named All Mail Actions from the BigBoss repository in Cydia.

If you're not willing to do any of these, you're going to have to delete each and every email individually. There's no other way.