arm64: mm: Drop pointless call to set_max_mapnr()

set_max_mapnr() is an empty stub function if CONFIG_NUMA=y, otherwise it
assigns to the 'max_mapnr' variable which is used to provide a generic
pfn_valid() implementation if CONFIG_MMU=n.

Since we don't support nommu on arm64, drop the pointless call to
set_max_mapnr() from mem_init().

Link: https://lore.kernel.org/r/130a50d7-92fd-31fa-261e-f73dadcb4fcf@redhat.com
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/mm/init.c

@@ -416,8 +416,6 @@ void __init mem_init(void)
 	else if (!xen_swiotlb_detect())
 		swiotlb_force = SWIOTLB_NO_FORCE;
 
-	set_max_mapnr(max_pfn - PHYS_PFN_OFFSET);
-
 	/* this will put all unused low memory onto the freelists */
 	memblock_free_all();