Auto merge of #1587 - lzutao:only-gha, r=RalfJung

build: Gate only on GHA: remove travis and appveyor ci config

It's been over a week since #1571 merged.
The GHA evaluation is successful.

This PR removes CI config of travis and appveyor.
Now MIRI gates on github actions CI only.

Will need <rust-lang/homu#109> to merge first,
otherwise bors will stuck.

Also, one may need to remove `continuous-integration/appveyor/pr` CI checks
in repo settings or appeveyor website.

Author: bors