PCI: xgene: Remove leftover pci_scan_child_bus() call

The changes in commit 9af275be ("PCI: xgene: Convert PCI scan API to
pci_scan_root_bus_bridge()") converted the xgene PCI host driver to
the new pci_scan_root_bus_bridge() bus scanning API but erroneously left
the existing pci_scan_child_bus() call in place which resulted in duplicate
PCI bus enumerations.

Remove the leftover pci_scan_child_bus() call to properly complete the API
conversion.

Fixes: 9af275be ("PCI: xgene: Convert PCI scan API to pci_scan_root_bus_bridge()")
Tested-by: Khuong Dinh <kdinh@apm.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: <stable@vger.kernel.org> # 4.13+
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Tanmay Inamdar <tinamdar@apm.com>