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.

internal/grid/debug.go

@@ -169,13 +169,13 @@ func dummyRequestValidate(r *http.Request) error {
 	return nil
 }
 
-func dummyTokenValidate(token, audience string) error {
-	if token == audience {
+func dummyTokenValidate(token string) error {
+	if token == "debug" {
 		return nil
 	}
-	return fmt.Errorf("invalid token. want %s, got %s", audience, token)
+	return fmt.Errorf("invalid token. want empty, got %s", token)
 }
 
-func dummyNewToken(audience string) string {
-	return audience
+func dummyNewToken() string {
+	return "debug"
 }