Remove .travis.yml as there are GitHub Actions already used for that

.travis.yml

@@ -1,27 +0,0 @@
-os: linux
-
-dist: trusty
-
-services: docker
-
-language: shell
-
-before_script:
-  - env | sort
-  - dir="${VARIANT}/"
-
-script:
-  - cd "$dir"
-  - travis_wait docker build --build-arg VERSION=latest -t ejabberd/ecs:travis-latest .
-  - docker images
-  - travis_retry docker run --name ejabberd -d -p 5222:5222 ejabberd/ecs && sleep 60
-  - docker ps
-  - docker logs ejabberd
-  - docker logs ejabberd | grep "Start accepting TCP connections" || exit 1
-
-notifications:
-  email: false
-
-env:
-  - VARIANT=ecs
-  - VARIANT=mix