Stop using Travis CI and AppVeyor

[yutannihilation]

It has been 3 months since we enabled GitHub Actions CI (#3862). While we still need to take care GitHub Actions CI time to time, it seems to catch the unintended changes reliably just as Travis CI and AppVeyor have been doing for years. So, I think it's okay to stop using Travis CI and AppVeyor now.

[karawoo]

This seems fine to me. We could also remove .travis.yml and appveyor.yml from .Rbuildignore, though I guess it doesn't really matter that much.

[yutannihilation]

We could also remove .travis.yml and appveyor.yml from .Rbuildignore

Thanks! I forgot to do this.

@thomasp85 @clauswilke
Do you have any concern to stop using Travis CI and AppVeyor? If not, I'll merge this (though it seems we need to change some setting on AppVayor to actually deactivate it)

[clauswilke]

I have no strong opinion one way or another.

[yutannihilation]

Thanks, Claus. I'm merging this.

[yutannihilation]

@thomasp85 @hadley
Could you disable the integrations with Travis CI and AppVeyor? We now rely only on GitHub Actions CI.