ci: Remove "ppc64le" machine

[buhtz]

I would like to remove the "ppc64le" machine from our TravisCI configuration without any replacement.

In short: It costs is much more higher than its value.

It was added once because an Apple user asked for. But to my research since 2006 Apple do not sell any ppc64le machines anymore. So their relevance for us is quit low.

On the other side this "ppc64le" machine on TravisCI is a big burden. It needs a lot of exceptional code to make it run and not false fail on some tests. To my experience in the last two yours maintaining BIT I never got a true failing test on ppc64le that didn't failed on the "amd64" machine, too. I might be a bit biased and emotional on this topic. I spent a lot of time and actually wasted it configuring the "ppc64le" machine and fixing "errors" that weren't actually real or significant errors.

On the long run we will clear up our test suite in a way so that GNU Linux distributions like Debian are able to run our test suite in their own build (debian#940319) pipeline which use several architectures.

And in context of sustainability not running this machine (~8 minutes per run) save time, energy, CO2 and money for TravisCI who providing this services to us for free.