xen/pciback: Remove unused pcistub_get_pci_dev

pcistub_get_pci_dev() was added in 2009 as part of:
commit 30edc14b ("xen/pciback: xen pci backend driver.")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Message-ID: <20250307004736.291229-1-linux@treblig.org>
Signed-off-by: Juergen Gross <jgross@suse.com>