all: cleanup unused code, part 1 (#10661)

Run `staticcheck` with `U1000` to find unused code. This cleans up about
a half of it. I'll do the other half separately to keep PRs manageable.

Updates #cleanup

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

net/tsdial/tsdial.go

@@ -196,18 +196,6 @@ func (d *Dialer) closeSysConn(id int) {
 	go c.Close() // ignore the error
 }
 
-func (d *Dialer) interfaceIndexLocked(ifName string) (index int, ok bool) {
-	if d.netMon == nil {
-		return 0, false
-	}
-	st := d.netMon.InterfaceState()
-	iface, ok := st.Interface[ifName]
-	if !ok {
-		return 0, false
-	}
-	return iface.Index, true
-}
-
 // peerDialControlFunc is non-nil on platforms that require a way to
 // bind to dial out to other peers.
 var peerDialControlFunc func(*Dialer) func(network, address string, c syscall.RawConn) error