Stop using Mitogen for tests

This removes some flakiness with newer Ansible and Debian releases,
at the expense of CI test speed.

.gitlab-ci.yml

@@ -21,7 +21,6 @@ variables:
       --passwords=${TEST_DIR}/passwords.yml
       --num-hosts=1
       ${LIBVIRT:+-e libvirt.remote_host=${LIBVIRT#*@} -e libvirt.remote_user=${LIBVIRT%@*}}
-      -e ansible_cfg.defaults.strategy=mitogen_linear ${MITOGEN:+-e ansible_cfg.defaults.strategy_plugins=${MITOGEN}/ansible_mitogen/plugins/strategy}
       ${APT_PROXY:+-e config.apt_proxy=${APT_PROXY}}
       $CREATE_ENV_VARS $BUILD_DIR