PCI/ASPM: Remove unused lists from struct pcie_link_state

ASPM does not make use of the children or link LIST_HEADs declared in
struct pcie_link_state and defined in alloc_pcie_link_state().  Therefore,
remove these lists.

No functional change intended.

Signed-off-by: Frederick Lawler <fred@fredlawl.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>