Revert "cmd/tailscale/cli: disallow --ssh on Synology"

This reverts commit 03e3e6abcd
in favor of #4785.

Change-Id: Ied65914106917c4cb8d15d6ad5e093a6299d1d48
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/up.go

@@ -440,10 +440,7 @@ func runUp(ctx context.Context, args []string) error {
 			return errors.New("--exit-node is " + notSupported)
 		}
 		if upArgs.netfilterMode != "off" {
-			return errors.New("--netfilter-mode values besides \"off\" are " + notSupported)
-		}
-		if upArgs.runSSH {
-			return errors.New("--ssh is " + notSupported)
+			return errors.New("--netfilter-mode values besides \"off\" " + notSupported)
 		}
 	}