Reset iPhone without iTunes or iCloud?
If, as you say, there is no iCloud account associated with that phone then restoring it using the DFU method should work. (This method applies when iPhone is on and plugged into a Mac with iTunes open)


Hold down the Power and Home buttons on the iPhone until the screen goes black
Continue holding Power and Home for two seconds after the screen goes black and then release the Power button, while still holding Home
iTunes will display a message saying it has detected a device in DFU mode (or recovery mode) and will give you an option to restore it. Release the buttons. 


If you're having issues with the actual download of the IPSW file itself then that'll be either your internet having issues with the download or Apple's software server having issues with delivering it.