remove unnecessary LRU for internode auth token (#20119)

removes contentious usage of mutexes in LRU, which
were never really reused in any manner; we do not
need it.

To trust hosts, the correct way is TLS certs; this PR completely
removes this dependency, which has never been useful.

```
0  0%  100%  25.83s 26.76%  github.com/hashicorp/golang-lru/v2/expirable.(*LRU[...])
0  0%  100%  28.03s 29.04%  github.com/hashicorp/golang-lru/v2/expirable.(*LRU[...])
```

Bonus: use `x-minio-time` as a nanosecond to avoid unnecessary
parsing logic of time strings instead of using a more
straightforward mechanism.

cmd/test-utils_test.go

@@ -83,6 +83,8 @@ func TestMain(m *testing.M) {
 		SecretKey: auth.DefaultSecretKey,
 	}
 
+	globalNodeAuthToken, _ = authenticateNode(auth.DefaultAccessKey, auth.DefaultSecretKey)
+
 	// disable ENVs which interfere with tests.
 	for _, env := range []string{
 		crypto.EnvKMSAutoEncryption,