canvas-container: Drop get_selected_icons_bounding_box API

This is not used anymore.

We only need one bounding box, which is provided by the new
nautilus_canvas_container_get_icon_bounding_box().

So remove unused API. We can always revert this if we need it.
1 job for antonioffix-menus-and-popovers in 7 minutes and 45 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Test
passed #5840
test

00:07:45

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