Review maintainer scripts

In prerm, don't drop the alternatives going through "active-theme" they are
the responsibility of desktop-base. In postinst, continue to create them
to update them in case the new kali theme provides the file that the
former theme did not.

Fix the check protecteding the call to plymouth-set-default to use a
version that actually existed in kali-themes even though we don't really
have an upgrade case where we wanted to call the command. But it will
make it easier to force a re-run in the future, in case we change the
name of the plymouth theme.

debian/kali-desktop-base.postinst

@@ -196,8 +196,9 @@ EOF
 
 fi
 
-# Set Kali as default plymouth theme
-if [ "${1}" = "configure" ] && dpkg --compare-versions "$2" lt "10.0.2+kali3"; then
+# Set Kali as default plymouth theme (on initial install and upgrade from
+# old version)
+if [ "${1}" = "configure" ] && dpkg --compare-versions "$2" lt "2019.4.0"; then
     if which plymouth-set-default-theme > /dev/null; then
         plymouth-set-default-theme kali
 	if which update-initramfs > /dev/null; then