How to get rid of iOS 7.x (downloaded but not installed) from 6.x As I've pointed it out in the question title, my 16 GiB iPhone 4s is getting out of memory due to the fact that it has self-downloaded iOS 7(.1.2) without I'd never asked for.

As long as it weights several GiB, there's any way to get rid of it ?
I've tried to navigate the file system through an app (for OS X) called "iFunBox", but there's no way to locate it.

Thanks in advice!
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.