clean up the platforms list inside platform.go

It's currently assumed we should release on all the platforms inside. However, there are 2 platforms fail this assumption: 

Name:      "ubuntu1604",

Arch:      ArchS390x,

This is only used for z race dectector. From the discussion with David, we should remove the test from z-series platform and remove this entry.

Name:      "ubuntu1604",

Arch:      ArchPpc64le, 

This platform is not used and should be removed.