Remove unused zfs-dkms.preinst script.

Remove unused zfs-dkms.preinst script.

Resolve this lintian warning:

W: zfs-dkms: maintainer-script-empty preinst
N:
N:    The maintainer script doesn't seem to contain any code other than
N:    comments and boilerplate (set -e, exit statements, and the case
N:    statement to parse options). While this is harmless in most cases, it is
N:    probably not what you wanted, may mean the package will leave
N:    unnecessary files behind until purged, and may even lead to problems in
N:    rare situations where dpkg would fail if no maintainer script was
N:    present.
N:
N:    If the package currently doesn't need to do anything in this maintainer
N:    script, it shouldn't be included in the package.
N:
N:    Severity: minor, Certainty: certain
N: