ubifs: remove unused ioctl flags GETFLAGS/SETFLAGS

In the ubifs, ubifs_fileattr_get and ubifs_fileattr_set
have been implemented, GETFLAGS and SETFLAGS ioctl are not
handled in filesystem's own ioctl helper. Additionally,
these flags' cases are not handled in ubifs's ioctl helper,
so we can remove them.

Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>