route: Remove two obsolete constants.

- DragonFly BSD removed RTM_OLDADD annd RTM_OLDDEL
  on Aug 4th, 2017 and are now gone in both master
  branch and RELEASE 5.6.

Author: tuxillo

route/defs_dragonfly.go

@@ -61,8 +61,6 @@ const (
 	sysRTM_REDIRECT   = C.RTM_REDIRECT
 	sysRTM_MISS       = C.RTM_MISS
 	sysRTM_LOCK       = C.RTM_LOCK
-	sysRTM_OLDADD     = C.RTM_OLDADD
-	sysRTM_OLDDEL     = C.RTM_OLDDEL
 	sysRTM_RESOLVE    = C.RTM_RESOLVE
 	sysRTM_NEWADDR    = C.RTM_NEWADDR
 	sysRTM_DELADDR    = C.RTM_DELADDR