Don't include model assertion name in FS path (LP: #1764541)

There can only be one model assertion, so we don't need to disambiguate them.
This also brings us in line with the behaviour of `snap prepare-image`, and
consistency is nice.

live-build/functions

@@ -422,7 +422,7 @@ snap_prepare_assertions() {
     local seed_dir="$CHROOT_ROOT/var/lib/snapd/seed"
     local snaps_dir="$seed_dir/snaps"
     local assertions_dir="$seed_dir/assertions"
-    local model_assertion="$assertions_dir/generic-classic.model"
+    local model_assertion="$assertions_dir/model"
     local account_key_assertion="$assertions_dir/generic.account-key"
     local account_assertion="$assertions_dir/generic.account"