travis: Remove CI integration
Removing Travis CI in favour of Azure Pipelines.
All tests previously tested by Travis are also configured in Azure.
Repos [1] and [2] were used to build Docker images for Travis, thus
they are no longer required for branches master and ipa-4-8.
Branches ipa-4-7 and ipa-4-6 don't have Azure pipelines configured,
so Travis will continue to be used by them.
1 - https://github.com/freeipa/ipa-docker-test-runner
2 - https://github.com/freeipa/freeipa-builder
Related: https://pagure.io/freeipa/issue/7323
Signed-off-by: Armando Neto <abiagion@redhat.com>
Reviewed-By: Francisco Trivino <ftrivino@redhat.com>