mm/mm_init.c: remove unneeded calc_memmap_size()

Nobody calls calc_memmap_size() now.

Link: https://lkml.kernel.org/r/20240325145646.1044760-6-bhe@redhat.com


Signed-off-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>