.gitlab-ci.d/cirrus: Remove the macos-15 job

.gitlab-ci.d/cirrus: Remove the macos-15 job

Cirrus-CI stopped providing the possibility to run macOS 15 jobs.
Quoting https://cirrus-ci.org/guide/macOS/ :

 "Cirrus CI Cloud only allows ghcr.io/cirruslabs/macos-runner:sonoma image ..."

If you still try to run a Sequoia image, it gets automatically "upgraded"
to Sonoma instead. So the macos-15 job in the QEMU CI now does not
make sense anymore, thus let's remove it.

Message-ID: <20241021124722.139348-1-thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>