route: remove RTM_LOCK on openbsd

RTM_LOCK is no longer defined on OpenBSD as of version 6.4, see
https://man.openbsd.org/OpenBSD-6.4/route.4 and
https://marc.info/?l=openbsd-tech&m=153018811429193&w=2

CL 419180 broke the build on openbsd/arm64 and openbsd/mips64 due to
RTM_LOCK not being defined on these platforms because these ports were
based on OpenBSD versions newer than 6.4. Earlier OpenBSD versions are
no longer supported since Go 1.15 per https://golang.org/wiki/OpenBSD

For golang/go#54035

Change-Id: I476a60ab2a082c7ddc763b5dbaf279ba47dcc1e1
Reviewed-on: https://go-review.googlesource.com/c/net/+/419937
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Tobias Klauser <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Run-TryBot: Tobias Klauser <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>

Author: tklauser

Diff for: route/sys_openbsd.go

@@ -73,7 +73,6 @@ func probeRoutingStack() (int, map[int]*wireFormat) {
 		unix.RTM_LOSING:     rtm,
 		unix.RTM_REDIRECT:   rtm,
 		unix.RTM_MISS:       rtm,
-		unix.RTM_LOCK:       rtm,
 		unix.RTM_RESOLVE:    rtm,
 		unix.RTM_NEWADDR:    ifam,
 		unix.RTM_DELADDR:    ifam,