xen: Clean up apic ipi interface

xen: Clean up apic ipi interface

Commit 5739711d82b00e5d89139724fc04acf80d987725 introduced the
implementation of the PV apic ipi interface. But there were some
odd things (it seems none of which cause really any issue but
maybe they should be cleaned up anyway):
 - xen_send_IPI_mask_allbutself (and by that xen_send_IPI_allbutself)
   ignore the passed in vector and only use the CALL_FUNCTION_SINGLE
   vector. While xen_send_IPI_all and xen_send_IPI_mask use the vector.
 - physflat_send_IPI_allbutself is declared unnecessarily. It is never
   used.

This patch tries to clean up those things.

Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>