How to get rid of iOS 7.x (downloaded but not installed) from 6.x
You cannot. iFunbox (or equivalent) will not have access to the root filesystem. You will need to jailbreak the device for this, and none exist for iOS 8 at this time.

However, note that even if you do remove the update, iOS will still periodically check for it and redownload the firmware at a later date. You would need to prevent this and there does exist a tweak (read: hack) to accomplish this, but again, you would need to jailbreak the device.