I don't use Backups in my PC, since I don't have enough hard drive space. How can I remove the "Restore Missing Files..." entry from my context menu?
The extension for this menu entry is in the package
% apt-file search /usr/lib/nautilus/extensions-3.0/libdeja-dup.so deja-dup: /usr/lib/nautilus/extensions-3.0/libdeja-dup.so
Therefore and as you don't use backups (really? You should read this.), a simple solution. Remove Déjà Dup from your system:
sudo apt-get remove deja-dup
and the menu entry will disappear.
Unfortunately this solution is like you having cancer and not wanting to take any chemo-therapy because it makes you feel bad. The better solution is to actually do backups, because otherwise we'll see each other again in a few years when your hard drive blows up and you've lost all your data!
It is unfortunate the Nautilus is not more user-friendly. The mentioned option cannot be removed neither form preferences nor from Nautilus actions tool. The easiest way to do it is to make changes to nautilus extensions. In the extensions folder rename:
After that restart nautilus:
If you ever want to use Backups again, no harm done. Just revert it to previous state.