arm64: mm: Drop pointless call to set_max_mapnr()

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>