wgengine/netstack: remove old unused handleSSH hook

It's leftover from an earlier Tailscale SSH wiring and I forgot to
delete it apparently.

Change-Id: I14f071f450e272b98d90080a71ce68ba459168d1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/netstack/netstack.go

@@ -127,10 +127,6 @@ type Impl struct {
 	connsOpenBySubnetIP map[netip.Addr]int
 }
 
-// handleSSH is initialized in ssh.go (on Linux only) to register an SSH server
-// handler. See https://github.com/tailscale/tailscale/issues/3802.
-var handleSSH func(logger.Logf, *ipnlocal.LocalBackend, net.Conn) error
-
 const nicID = 1
 const mtu = tstun.DefaultMTU