LoongArch: Fix arch_remove_memory() undefined build error

LoongArch: Fix arch_remove_memory() undefined build error

The kernel build error when unslected CONFIG_MEMORY_HOTREMOVE because
arch_remove_memory() is needed by mm/memory_hotplug.c but undefined.

Some build error messages like:

 LD      vmlinux.o
 MODPOST vmlinux.symvers
 MODINFO modules.builtin.modinfo
 GEN     modules.builtin
 LD      .tmp_vmlinux.kallsyms1
loongarch64-linux-gnu-ld: mm/memory_hotplug.o: in function `.L242':
memory_hotplug.c:(.ref.text+0x930): undefined reference to `arch_remove_memory'
make: *** [Makefile:1169：vmlinux] 错误 1

Removed CONFIG_MEMORY_HOTREMOVE requirement and rearrange the file refer
to the definitions of other platform architectures.

Signed-off-by: Yupeng Li <liyupeng@zbhlos.com>
Signed-off-by: Caicai <caizp2008@163.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>