mm/mm_init.c: remove arch_reserved_kernel_pages()

Since the current calculation of calc_nr_kernel_pages() has taken into
consideration of kernel reserved memory, no need to have
arch_reserved_kernel_pages() any more.

Link: https://lkml.kernel.org/r/20240325145646.1044760-7-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>