tstest/integration: remove vestigial env var set in tests

TS_CONTROL_IS_PLAINTEXT_HTTP no longer does anything as of
8fd471ce57

Updates #13597

Change-Id: I32ae7f8c5f2a2632e80323b1302a36295ee00736
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/integration/integration.go

@@ -684,7 +684,6 @@ func (n *TestNode) StartDaemonAsIPNGOOS(ipnGOOS string) *Daemon {
 		cmd.Args = append(cmd.Args, "--config="+n.configFile)
 	}
 	cmd.Env = append(os.Environ(),
-		"TS_CONTROL_IS_PLAINTEXT_HTTP=1",
 		"TS_DEBUG_PERMIT_HTTP_C2N=1",
 		"TS_LOG_TARGET="+n.env.LogCatcherServer.URL,
 		"HTTP_PROXY="+n.env.TrafficTrapServer.URL,