UBUNTU: [Packaging] dkms-build--nvidia-N -- clean up unsigned ko files

UBUNTU: [Packaging] dkms-build--nvidia-N -- clean up unsigned ko files

BugLink: https://bugs.launchpad.net/bugs/1834479
We currently end up with duplicate ko files under /lib/modules,
one signed and one unsigned. depmod can end up prefering
the unsigned module, so remove it after generating the signed
copy.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Andy Whitcroft <andy.whitcroft@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>