travis-ci/gitlab-ci: remove Ubuntu 18.10 Cosmic

Found that on 19.02.2020 APT repositories with packages
for Ubuntu 18.10 Cosmic were removed from Ubuntu archive:

 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.

Also found the half a year old message about Ubuntu 18.10
Cosmic EOL:
 https://fridge.ubuntu.com/2019/07/19/ubuntu-18-10-cosmic-cuttlefish-end-of-life-reached-on-july-18-2019/

Removed the Ubuntu 18.10 Cosmic from gitlab-ci and
travis-ci testings.

(cherry picked from commit 961e8c5f)