Fix logic to ensure snapd is seeded in core18-only images

The _snap_post_process function is meant to install snapd if core18 is the
only core snap installed or removed snapd if core is installed and snapd
was not explicitly installed.  But the current logic in _snap_preseed
will never call _snap_post_process.  $core_name will never be empty
with the existing logic, but even if it were that would only be for the
'core' snap and we'd miss using the 'core18' logic that pulls in snapd.
Given the case statement in _snap_post_process can handle doing the
right thing given any snap we can just call it unconditionally.

live-build/functions

@@ -536,11 +536,7 @@ EOF
     echo -n "    file: " >> $seed_yaml
     (cd $snaps_dir; ls -1 ${SNAP_NAME}_*.snap) >> $seed_yaml
 
-    # If $core_snap is the empty string then SNAP itself *may be* a core snap,
-    # and we run some post-processing logic.
-    if [ -z "$core_snap" ]; then
-        _snap_post_process $CHROOT_ROOT $SNAP_NAME
-    fi
+    _snap_post_process $CHROOT_ROOT $SNAP_NAME
 }
 
 snap_prepare_assertions() {