Remove HTTP collector check on identifier label

This commit removes the logic that checks for an identifier label in the
http collector config. It also removes the documentation on the README
that mentions that the metric has to be in an old format, removed in
the #219.

Fixes #331

Signed-off-by: Jonathan Juares Beber <jonathanbeber@gmail.com>

pkg/collector/http_collector_test.go

@@ -85,7 +85,7 @@ func makeTestHTTPCollectorConfig(endpoint, aggregator string) *MetricConfig {
 			Metric: v2beta2.MetricIdentifier{
 				Name: "test-metric",
 				Selector: &v1.LabelSelector{
-					MatchLabels: map[string]string{identifierLabel: "test-metric"},
+					MatchLabels: map[string]string{"type": HTTPJSONPathType},
 				},
 			},
 		},