Remove (useless) flaky test

[mbusi]

The test was flaky because it was relying on a weird race condition moslty related to the order the processes shutdown when closing a channel; sometimes it failed with

  1) test basic consume cancel exits the process when channel is closed (BasicTest)
     test/basic_test.exs:95
     match (=) failed
     code:  assert {:normal, _} = catch_exit(Basic.cancel(meta[:chan], consumer_tag))
     left:  {:normal, _}
     right: {
              :noproc,
              {:gen_server, :call, [#PID<0.372.0>, {:call, {:"basic.cancel", "amq.ctag-7S2NmqLj18nRwk_-O_tQ1A", false}, :none, #PID<0.360.0>}, 70000]}
            }
     stacktrace:
       test/basic_test.exs:103: (test)

This can be easily reproduced by simply adding a

:timer.sleep(100)

after closing the channel

I'd argue that this test was useless anyway (I still don't understand what the test purpose was) so I simply removed it 🤷