cmd/tailscale,ipn,net/netutil: remove rp_filter strict mode warnings (#18863)

PR #18860 adds firewall rules in the mangle table to save outbound packet
marks to conntrack and restore them on reply packets before the routing
decision. When reply packets have their marks restored, the kernel uses
the correct routing table (based on the mark) and the packets pass the
rp_filter check.

This makes the risk check and reverse path filtering warnings unnecessary.

Updates #3310
Fixes tailscale/corp#37846

Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>

client/local/local.go

@@ -818,25 +818,6 @@ func (lc *Client) CheckUDPGROForwarding(ctx context.Context) error {
 	return nil
 }
 
-// CheckReversePathFiltering asks the local Tailscale daemon whether strict
-// reverse path filtering is enabled, which would break exit node usage on Linux.
-func (lc *Client) CheckReversePathFiltering(ctx context.Context) error {
-	body, err := lc.get200(ctx, "/localapi/v0/check-reverse-path-filtering")
-	if err != nil {
-		return err
-	}
-	var jres struct {
-		Warning string
-	}
-	if err := json.Unmarshal(body, &jres); err != nil {
-		return fmt.Errorf("invalid JSON from check-reverse-path-filtering: %w", err)
-	}
-	if jres.Warning != "" {
-		return errors.New(jres.Warning)
-	}
-	return nil
-}
-
 // SetUDPGROForwarding enables UDP GRO forwarding for the main interface of this
 // node. This can be done to improve performance of tailnet nodes acting as exit
 // nodes or subnet routers.