staging: erofs: remove a redundant marco in xattr

There is no need to '#if CONFIG_EROFS_FS_XATTR' in xattr.c,
let's remove it.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>