You need to reinstall the build you have deleted - put the iPod into disk mode (the manual tells you how) and then extract the build (the manual tells you how).
Incidently, you can dual boot Rockbox and the Apple firmware (the manual tells you how), so there is no need to uninstall Rockbox to watch videos in the original firmware.
If you so wish to uninstall Rockbox then, you guessed it, the manual tells you how.