Scrape: Remove JSON from Accept request header

JSON is no longer supported as an exposition format [1] [2] [3]. Remove
it from the `Accept` header added to requests when scraping targets.

[1]: https://github.com/prometheus/prometheus/blob/master/CHANGELOG.md#100--2016-07-18
[2]: https://prometheus.io/docs/instrumenting/exposition_formats/#historical-versions
[3]: https://docs.google.com/document/d/1ZjyKiKxZV83VI9ZKAXRGKaUKK2BIWCT7oiGBKDBpjEY/edit?usp=sharing

retrieval/scrape.go

@@ -308,7 +308,7 @@ type targetScraper struct {
 	client *http.Client
 }
 
-const acceptHeader = `application/vnd.google.protobuf;proto=io.prometheus.client.MetricFamily;encoding=delimited;q=0.7,text/plain;version=0.0.4;q=0.3,application/json;schema="prometheus/telemetry";version=0.0.2;q=0.2,*/*;q=0.1`
+const acceptHeader = `application/vnd.google.protobuf;proto=io.prometheus.client.MetricFamily;encoding=delimited;q=0.7,text/plain;version=0.0.4;q=0.3,*/*;q=0.1`
 
 func (s *targetScraper) scrape(ctx context.Context, ts time.Time) (model.Samples, error) {
 	req, err := http.NewRequest("GET", s.URL().String(), nil)