Revert "Add qt6 CI support"

This wasn't tested and it can't possibly work for now because
plasma-framework has no support for Qt6 at the moment.

This reverts commit 57b0485f.

.gitlab-ci.yml

@@ -4,4 +4,3 @@
 include:
   - https://invent.kde.org/sysadmin/ci-utilities/raw/master/gitlab-templates/linux.yml
   - https://invent.kde.org/sysadmin/ci-utilities/raw/master/gitlab-templates/freebsd.yml
-  - https://invent.kde.org/sysadmin/ci-utilities/raw/master/gitlab-templates/linux-qt6.yml