textview: Properly delete anchored children

textview: Properly delete anchored children

When we remove anchors with widgets from the text
buffer, we used to call gtk_widget_destroy(), which
indirectly called gtk_container_remove() which cleared
the child properly. When gtk_widget_destroy() was
removed, we replaced the calls with gtk_widget_unparent(),
but that is not enough. Explicitly call
gtk_text_view_remove() instead - we know the parent
is a text view.