LoongArch: Remove unused code to avoid build warning

LoongArch: Remove unused code to avoid build warning

After commit feee6b2989165631b1 ("mm/memory_hotplug: shrink zones when
offlining memory"), __remove_pages() doesn't need the "zone" parameter
so the "page" variable is also unused. Remove the unused code to avoid
such build warning:

arch/loongarch/mm/init.c: In function 'arch_remove_memory':
arch/loongarch/mm/init.c:134:22: warning: variable 'page' set but not used [-Wunused-but-set-variable=]
  134 |         struct page *page = pfn_to_page(start_pfn);

Cc: <stable@vger.kernel.org>
Reviewed-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>