I spent a good part of this past weekend researching this and trying MANY different things to get rid of the update file which now resides (and continues to reside) on the iDevice (iPad mini in my case)...the update was pushed to my iPad mini without my knowledge & without me choosing to do so. It can be removed if the device is restored as new, BUT it will again be downloaded to the device when:
1. It's connected to the PC via USB (in other words, the iDevice is externally powered by the PC) which has a working internet connection
2. It's connected to the charger and the iDevice itself has a valid internet connection.
The ONLY way to remove it and *keep it off of the device* is to restore the device as new, restore all your Apps, music etc. with the internet disabled (on both the device and the PC), then NEVER allow it to connect to the internet.
Your only recourse is to ignore the update (even though it's taking that huge chunk of space) and do NOT push the Install button under Settings/General/Software Update
This is really bad practice, forcing the update to download onto the device over and over again despite the user not wanting it nor wishing to update it. IMO, this is borderline on being a virus since you can't remove it under normal means and if you DO remove it, it is downloaded to your device again (and again).