metrics: remove wrong jenkins_exporter_errors{type=http_requests_total} metric

The jenkins_http_requests_total and jenkins_exporter_errors metrics are mixed
up.
A metric called jenkins_exporter_errors with the label http_requests_total is
incremented for every http requests.
The name implied it only counts errors but this is not the case.
The documented metric called jenkins_http_requests_total is not recorded at all.

Remove jenkins_exporter_errors metrics with the http_requests_total label.
Remove mentions of the jenkins_http_requests_total metrics in the README.

If it turns out that there is a use case for the jenkins_http_requests_total
metric it can be added back.

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http: do not use buffer when draining and discarding http body

Use io.Copy with io.Discard instead of io.ReadAll to drain the whole http body
when an error occurred.

This does not write the body unnecessary to a buffer in memory that will never
be used.