PCI: Remove unused pcie_aspm_enabled()

My philosophy is unused code is dead code.  And dead code is subject to bit
rot and is a likely source of bugs.  Use it or lose it.

This reverts part of 3e1b16002af2 ("ACPI/PCI: PCIe ASPM _OSC support
capabilities called when root bridge added"), removing this interface:

    pcie_aspm_enabled()

[bhelgaas: split to separate patch]
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: Andrew Patterson <andrew.patterson@hp.com>