travis: Remove s390x build

The build didn't find too many issues, and they are generally only
endianness related, so easy to fix if they do make it into the main
developement branch.

With travis migrating its service, it fails too often intermittently. It
might be more appropriate to run it manually once before every release.

.travis.yml

@@ -72,13 +72,6 @@ jobs:
         FILE_ENV="./ci/test/00_setup_env_arm.sh"
         QEMU_USER_CMD=""
 
-    - stage: test
-      name: 'S390x  [GOAL: install]  [buster]  [unit tests, functional tests]'
-      arch: s390x  # Can disable QEMU_USER_CMD and run the tests natively without qemu
-      env: >-
-        FILE_ENV="./ci/test/00_setup_env_s390x.sh"
-        QEMU_USER_CMD=""
-
     - stage: test
       name: 'Win64  [GOAL: deploy]  [unit tests, no gui, no boost::process, no functional tests]'
       env: >-