arm/xen: Remove definiition of virt_to_pfn in asm/xen/page.h

virt_to_pfn has been defined in asm/memory.h by the commit e26a9e00 "ARM: Better
virt_to_page() handling"

This will result of a compilation warning when CONFIG_XEN is enabled.

arch/arm/include/asm/xen/page.h:80:0: warning: "virt_to_pfn" redefined [enabled by default]
 #define virt_to_pfn(v)          (PFN_DOWN(__pa(v)))
 ^
In file included from arch/arm/include/asm/page.h:163:0,
                 from arch/arm/include/asm/xen/page.h:4,
                 from include/xen/page.h:4,
                 from arch/arm/xen/grant-table.c:33:

The definition in memory.h is nearly the same (it directly expand PFN_DOWN),
so we can safely drop virt_to_pfn in xen include.
Signed-off-by: Julien Grall <julien.grall@linaro.org>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

arch/arm/include/asm/xen/page.h

@@ -77,7 +77,6 @@ static inline xpaddr_t machine_to_phys(xmaddr_t machine)
 }
 /* VIRT <-> MACHINE conversion */
 #define virt_to_machine(v)	(phys_to_machine(XPADDR(__pa(v))))
-#define virt_to_pfn(v)          (PFN_DOWN(__pa(v)))
 #define virt_to_mfn(v)		(pfn_to_mfn(virt_to_pfn(v)))
 #define mfn_to_virt(m)		(__va(mfn_to_pfn(m) << PAGE_SHIFT))