mm: move is_pageblock_removable_nolock() to mm/memory_hotplug.c

mm: move is_pageblock_removable_nolock() to mm/memory_hotplug.c

is_pageblock_removable_nolock() is not used outside of
mm/memory_hotplug.c.  Move it next to unique caller
is_mem_section_removable() and make it static.

Remove prototype in <linux/memory_hotplug.h> to silence gcc warning (W=1):

  mm/page_alloc.c:7704:6: warning: no previous prototype for `is_pageblock_removable_nolock' [-Wmissing-prototypes]

Link: http://lkml.kernel.org/r/20180509190001.24789-1-malat@debian.org
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Suggested-by: Michal Hocko <mhocko@kernel.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>