MM-62365: Remove unused field from MetricSample (#30991)

The Timestamp field was not used at all. Therefore
removing it.

https://mattermost.atlassian.net/browse/MM-62365

```release-note
NONE
```

server/channels/app/metrics_test.go

@@ -64,7 +64,7 @@ func TestMobileMetrics(t *testing.T) {
 		var start float64 = 125
 		end := start + float64(len(ttcc))
 		// Precheck
-		for i, tc := range ttcc {
+		for _, tc := range ttcc {
 			actualMetric, err := tc.histogramVec.GetMetricWith(prometheus.Labels{"platform": platform})
 			require.NoError(t, err)
 			require.NoError(t, actualMetric.(prometheus.Histogram).Write(m))
@@ -73,7 +73,6 @@ func TestMobileMetrics(t *testing.T) {
 			histograms = append(histograms, &model.MetricSample{
 				Metric: tc.metricName,
 				Value:  tc.elapsed,
-				Timestamp: start + float64(i),
 			})
 		}
 

server/public/model/metrics.go

@@ -86,7 +86,6 @@ var (
 type MetricSample struct {
 	Metric MetricType        `json:"metric"`
 	Value  float64           `json:"value"`
-	Timestamp float64           `json:"timestamp,omitempty"`
 	Labels map[string]string `json:"labels,omitempty"`
 }