xen: Remove hanging references to CONFIG_XEN_PLATFORM_PCI

In 5fbdc103 the XEN_PLATFORM_PCI config
option was removed, but references in header files remained. Clean up
those references.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>