printk/index: Fix -Wunused-function warning

If CONFIG_MODULES is n, we got this:

kernel/printk/index.c:146:13: warning: ‘pi_remove_file’ defined but not used [-Wunused-function]

Move it inside #ifdef block to fix this warning.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210804130105.18732-1-yuehaibing@huawei.com