test: remove test-http-exit-delay

test-http-exit-delay has a flaky history. Examination of the bug it was
written to find suggests that the test should be removed.

* The test is trying to find a delay of up to 1 second, but the delay
may also be much smaller than that. So the test will not catch the bug
all the time. It is therefore flaky when the bug exists.

* Experience has shown that the test is flaky as well when the bug is
absent in the code because it can sometimes take slower devices (such as
the Raspberry Pi devices that we have in continuous integration) more
than the allowed one second to run. Increasing the timeout for those
devices will make the test pass but will also mean that the test isn't
really testing anything because the delay it is trying to catch was a
delay of up to one second.

I don't think this is an appropriate test to run once on CI. If there is
to be a test for the issue in question, it should be a benchmark test
that is run a large number of times. We don't really have such tests in
CI yet

I would argue that this test is actively a problem. It does not reliably
catch the issue it is supposed to catch, nor can it likely be made to do
so. (To do so would likely require converting it to a benchmarking test
as previously described. We don't run those in CI, at least not at this
time.)

Because this test will have both false positives and false negatives,
especially on the slower devices, it contributes to a culture of
dismissing failed tests. It does not reliably identify an issue nor does
it reliably pass on a working code base. This test should be removed.

Ref: #4277
PR-URL: #4786
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>