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 https://source.puri.sm/Librem5/libhandy/issues/191

(cherry picked from commit 1c2d9cac)
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