mm/Kconfig: remove redundant arch depend for memory hotplug

MEMORY_HOTPLUG already depends on ARCH_ENABLE_MEMORY_HOTPLUG which is
selected by the supported architectures, so the following arch depend is
unnecessary.

Signed-off-by: Yang Shi <yang.shi@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>