arch/x86/mm/init_64.c: fix build warning when CONFIG_MEMORY_HOTREMOVE=n

arch/x86/mm/init_64.c: fix build warning when CONFIG_MEMORY_HOTREMOVE=n

There is a warning while building kernel with
CONFIG_MEMORY_HOTPLUG=y && CONFIG_MEMORY_HOTREMOVE=n:
arch/x86/mm/init_64.c:1024: warning:kernel_physical_mapping_remove defined but not used

So move kernel_physical_mapping_remove() into "#ifdef
CONFIG_MEMORY_HOTREMOVE" block

Signed-off-by: Jianguo Wu <wujianguo@huawei.com>
Cc: Wen Congyang <wency@cn.fujitsu.com>
Cc: Tang Chen <tangchen@cn.fujitsu.com>
Cc: Liujiang <jiang.liu@huawei.com>
Cc: qiuxishi <qiuxishi@huawei.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>