Remove concurrencytest

Remove concurrencytest

While our version is better, it is tricky to use it when we are trying
to package things with pip. Drop it.

Somewhat reduced functionality is provided by the upstream version[1],
along with a rather annoying message each time it is used[2] [3].

[1] pip install concurrencytest
[2] https://github.com/cgoldberg/concurrencytest/issues/12
[3] https://github.com/cgoldberg/concurrencytest/pull/14

Signed-off-by: Simon Glass <sjg@chromium.org>