Remove unused continuous integration configs

[cclauss]

Signed-off-by: Christian Clauss [email protected]

AppVeyor, CircleCI, and Travis CI all used to be the test engines for this repo but they are now all disabled and replaced by Azure Pipelines, et al. This PR removes unused continuous integration configs and scripts which may distract contributors.

Please provide an overview of what this pull request does, and reference any relevant issues that are addressed. Once submitted you are encouraged to seek review of the code, and to check the continuous integration results.

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