Remove first contentful paint cdp metric

We're now getting web vital metrics which are calculated by the JS
web vital script. It also contains first contentful paint FCP metric.
The metric coming from the JS lib also comes back with a rating. We
can't correlate that with the current CDP FCP. To keep web vital
metrics source consistent, we will remove the existing CDP FCP and opt
to work with the one coming from the JS lib.

common/frame_session.go

@@ -808,7 +808,6 @@ func (fs *FrameSession) onPageLifecycle(event *cdppage.EventLifecycleEvent) {
 		"load":                 fs.k6Metrics.BrowserLoaded,
 		"DOMContentLoaded":     fs.k6Metrics.BrowserDOMContentLoaded,
 		"firstPaint":           fs.k6Metrics.BrowserFirstPaint,
-		"firstContentfulPaint": fs.k6Metrics.BrowserFirstContentfulPaint,
 		"firstMeaningfulPaint": fs.k6Metrics.BrowserFirstMeaningfulPaint,
 	}
 

examples/multiple-scenario.js

@@ -18,7 +18,7 @@ export const options = {
   },
   thresholds: {
     browser_dom_content_loaded: ['p(90) < 1000'],
-    browser_first_contentful_paint: ['max < 1000'],
+    browser_first_meaningful_paint: ['max < 1000'],
     checks: ["rate==1.0"]
   }
 }

k6ext/metrics.go

@@ -19,7 +19,6 @@ const (
 type CustomMetrics struct {
 	BrowserDOMContentLoaded     *k6metrics.Metric
 	BrowserFirstPaint           *k6metrics.Metric
-	BrowserFirstContentfulPaint *k6metrics.Metric
 	BrowserFirstMeaningfulPaint *k6metrics.Metric
 	BrowserLoaded               *k6metrics.Metric
 
@@ -60,8 +59,6 @@ func RegisterCustomMetrics(registry *k6metrics.Registry) *CustomMetrics {
 			"browser_dom_content_loaded", k6metrics.Trend, k6metrics.Time),
 		BrowserFirstPaint: registry.MustNewMetric(
 			"browser_first_paint", k6metrics.Trend, k6metrics.Time),
-		BrowserFirstContentfulPaint: registry.MustNewMetric(
-			"browser_first_contentful_paint", k6metrics.Trend, k6metrics.Time),
 		BrowserFirstMeaningfulPaint: registry.MustNewMetric(
 			"browser_first_meaningful_paint", k6metrics.Trend, k6metrics.Time),
 		BrowserLoaded: registry.MustNewMetric(