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/msg.go

@@ -262,14 +262,9 @@ type connectReq struct {
 	Token string
 }
 
-// audience returns the audience for the connect call.
-func (c *connectReq) audience() string {
-	return fmt.Sprintf("%s-%d", c.Host, c.Time.Unix())
-}
-
 // addToken will add the token to the connect request.
 func (c *connectReq) addToken(fn AuthFn) {
-	c.Token = fn(c.audience())
+	c.Token = fn()
 }
 
 func (connectReq) Op() Op {