CI: Remove Travis builds

[edalex-ian]

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Description of change

We've now migrated to GitHub Actions with good results after running for 1 to 2 months. So as per the CDM decision, let's remove Travis (starting here on develop only - will keep on older stable branches etc).

[edalex-ian]

Drats! And GHA then goes to fail with it's unique case I've seen a couple of times.

On uploading the artefacts, it sometimes fails, which means the tests then can't get access to the installer to test against. But for some reason, even when artefact upload fails, the task is marked as successful but you see this in the logs:

Retry limit has been reached for chunk at offset 398458880 to https://pipelines.actions.githubusercontent.com/iRC5GUUXoTDFYBfBPWte0CZHpnFt0VgNRq4iivqbrN4Qg9rcsB/_apis/resources/Containers/2712561?itemPath=Artefacts%2Fartefacts.tar
##[warning]Aborting upload for /home/runner/work/openEQUELLA/openEQUELLA/artefacts.tar due to failure
Total size of all the files uploaded is 398458880 bytes
Finished uploading artifact Artefacts. Reported size is 398458880 bytes. There were 1 items that failed to upload
Artifact upload has finished successfully!

Anyway, I've restarted the build. 😞

If this continues in the future, we can raise an issue against the action - or it seems like it could be actions/upload-artifact#71

[edalex-ian]

Looks okay now, just failed a known flaky test. HierarchyTopicTest.

Probably fine to merge.

[PenghaiZhang]

I think we can also remove travis related files in autotest such as travis.conf and yaml files (e.g. testng-travis-contribution.yaml)

[edalex-ian]

Good point @PenghaiZhang , I'll do that now(ish).

[edalex-ian]

If I get green on GHA and CodeBuild, I'll merge on in.