Remove obsolete CI.

[veelo]

1. The AppVeyor CI is broken in the sense that failures are not detected, ci.bat always ends with exit value 0. Disabling AppVeyor requires admin rights.
2. It seems we have stopped using Travis-CI.

GitHub Actions currently covers all platforms, and makes other CI systems obsolete.

When AppVeyor has been disabled, the following files can be removed:

.travis.yml
appveyor.yml
ci.bat
ci.sh