arm64: mm: Fix compilation when CONFIG_MEMORY_HOTPLUG is not enabled

remove_pagetable() should only be called when CONFIG_MEMORY_HOTPLUG
is enabled to clean up the page table entries created when memory
was hot-added.

Change-Id: I38cf42673a7637dac62d13a3083b0963c8a435b3
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>