gitlab-ci.d/buildtest: Remove ppc-softmmu from the clang-system job

gitlab-ci.d/buildtest: Remove ppc-softmmu from the clang-system job

We are also compile-testing ppc64-softmmu with clang in the "tsan-build"
job, and ppc64-softmmu covers pretty much the same code as ppc-softmmu,
so we should not lose much test coverage here by removing ppc-softmmu
from the "clang-system" job.

Message-Id: <20230130104446.1286773-2-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>