arm64: remove unused ARCH_PFN_OFFSET

This is only relevant to the FLATMEM memory model, which isn't an option
since commit 782276b ("arm64: Force SPARSEMEM_VMEMMAP as the only
memory management model").

Signed-off-by: Omar Sandoval <osandov@fb.com>
Acked-by: Will Deacon <will@kernel.org>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: osandov

arch/arm64/include/asm/memory.h

@@ -392,7 +392,6 @@ static inline unsigned long virt_to_pfn(const void *kaddr)
  *  virt_to_page(x)	convert a _valid_ virtual address to struct page *
  *  virt_addr_valid(x)	indicates whether a virtual address is valid
  */
-#define ARCH_PFN_OFFSET		((unsigned long)PHYS_PFN_OFFSET)
 
 #if defined(CONFIG_DEBUG_VIRTUAL)
 #define page_to_virt(x)	({						\