PCI: fix gathering of PCI domains

Commit 1e57af5 removed macros HWLOC_HAVE_PCIDEV_DOMAIN
and HWLOC_HAVE_LIBPCIACCESS. Don't require them when
gathering PCI domain, otherwise we'll always get 0.

Thanks to James Custer for the fix.
PCI: remove dead code for HWLOC_HAVE_PCI_FIND_CAP

This was pciutils-specific.

Thanks to James Custer for the patch.