.shippable: remove the last bits

.shippable: remove the last bits

Shippable is about to sunset in May 2021 [1] and we had already moved
a chunk of the crossbuilds to GitLab. We already cross build
mips-softmmu targets since:

  6bcb5fc0f7 ("gitlab-ci: Add cross-compiling build tests")

and x86 is very well covered.

[1]: https://blog.shippable.com/the-next-step-in-the-evolution-of-shippable-jfrog-pipelines

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210211122750.22645-7-alex.bennee@linaro.org>