Require brand:model to be passed to snap_prepare_assertions

We only need the default in one place, so remove it from all but the top-level
function.

live-build/functions

@@ -415,9 +415,9 @@ EOF
 snap_prepare_assertions() {
     # Configure basic snapd assertions
     local CHROOT_ROOT=$1
-    # Optional. If set, should be a colon-separated string of brand:model to be
+    # A colon-separated string of brand:model to be used for the image's model
-    # used for the image's model assertion
+    # assertion
-    local CUSTOM_BRAND_MODEL=${2:-generic:generic-classic}
+    local CUSTOM_BRAND_MODEL=$2
 
     local seed_dir="$CHROOT_ROOT/var/lib/snapd/seed"
     local snaps_dir="$seed_dir/snaps"