simplify function, some marginal perf gain

internal/experiment/openvpn/targets.go

@@ -31,21 +31,10 @@ func sampleN(a []string, n int) []string {
 	if n > len(a) {
 		n = len(a)
 	}
-
-	sampled := make([]string, 0)
-
-	// Use a map to track indices we've already selected to avoid duplicates
-	picked := make(map[int]struct{})
-
-	for len(sampled) < n {
-		idx := rand.Intn(len(a)) // Random index
-		if _, exists := picked[idx]; !exists {
-			sampled = append(sampled, a[idx])
-			picked[idx] = struct{}{} // Mark index as used
-		}
-	}
-
-	return sampled
+	rand.Shuffle(len(a), func(i, j int) {
+		a[i], a[j] = a[j], a[i]
+	})
+	return a[:n]
 }
 
 // resolveOONIAddresses returns a max of maxDefaultOONIAddresses after