travis-ci: remove Ubuntu Cosmic

It reached EOL and the CI job fails with the following output:

 | sudo apt-get update > /dev/null
 | E: The repository 'http://security.ubuntu.com/ubuntu cosmic-security Release' does not have a Release file.
 | E: The repository 'http://archive.ubuntu.com/ubuntu cosmic Release' does not have a Release file.
 | E: The repository 'http://archive.ubuntu.com/ubuntu cosmic-updates Release' does not have a Release file.
 | E: The repository 'http://archive.ubuntu.com/ubuntu cosmic-backports Release' does not have a Release file.

Author: Totktonada

.travis.yml

@@ -89,8 +89,6 @@ jobs:
       env: OS=ubuntu DIST=xenial
     - <<: *packpack
       env: OS=ubuntu DIST=bionic
-    - <<: *packpack
-      env: OS=ubuntu DIST=cosmic
     - <<: *packpack
       env: OS=ubuntu DIST=disco
     - <<: *packpack