Bye bye httpbin.org, you've served us well.

The service has been down sporadically for a few months already and there doesn't appear to be a clear path forward (see postmanlabs/httpbin#702 and others). I've replaced most occurrences with httpbingo.org as a drop-in replacement where possible.

We've introduced this a while back via reactphp/http-client#109 before merging the HTTP client with #368 (see also reactphp/socket#105 and #244) and haven't touched this ever since. These tests don't provide a lot of value nowadays given that this is a rather high-level test and the underlying logic is already covered by our unit test suite. Accordingly, removing these tests has no impact on our code coverage.

See also #480, #481 and others, as we'll likely eliminate the legacy Client class in the future