shadow-helper: Don't set style context parent

Even though foreign drawing uses a temporary GtkStyleContext, calling
gtk_style_context_set_parent() adds all its CSS nodes to the global
hierarchy, and then they aren't cleaned up. This shouldn't happen.

Fixes Librem5/libhandy#191
12 jobs for wip/exalm/nodes in 22 minutes and 19 seconds (queued for 23 seconds)