x86/mm: Remove the now redundant N_MEMORY check

In commit

  f70029bb

 ("mm, memory_hotplug: drop CONFIG_MOVABLE_NODE")

the dependency on CONFIG_MOVABLE_NODE was removed for N_MEMORY.
Before, CONFIG_HIGHMEM && !CONFIG_MOVABLE_NODE could make (N_MEMORY ==
N_NORMAL_MEMORY) be true.

After that commit, N_MEMORY cannot be equal to N_NORMAL_MEMORY. So the
conditional check in paging_init() is not needed anymore, remove it.

 [ bp: Massage. ]

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Link: https://lkml.kernel.org/r/20200311011823.27740-1-bhe@redhat.com