ui/gtk3: Fix radio buttons on Property Panel
ui/gtk3: Fix radio buttons on Property Panel

Use gtk_container_remove() instead g_object_unref() because
if an widget has a parent, it's not destroyed and the signal is not
sent to the parent since the parent was destroyed.

R=shawn.p.huang@gmail.com

Review URL: https://codereview.appspot.com/302650043

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