Remove RTNH_F_EMPTY

The `RTNH_F_EMPTY` does not exist in kernel or /usr/include and is
invalid for bitmask.

Signed-off-by: Gris Ge <[email protected]>

Author: cathay4t

src/rtnl/route/nlas/next_hops.rs

@@ -15,7 +15,6 @@ use netlink_packet_utils::{
 bitflags! {
     #[non_exhaustive]
 pub struct NextHopFlags: u8 {
-        const RTNH_F_EMPTY = 0;
         const RTNH_F_DEAD = constants::RTNH_F_DEAD;
         const RTNH_F_PERVASIVE = constants::RTNH_F_PERVASIVE;
         const RTNH_F_ONLINK = constants::RTNH_F_ONLINK;