CI: Remove linux-qt6 build

This commit follows the recommendation of our system administrator
Ben Cooksley to remove the linux-qt6 build from the CI for projects
which already have linux-qt6-next and FreeBSD or Windows builds
which also use qt6 (non-next).

See https://mail.kde.org/pipermail/kde-devel/2025-April/003464.html

Author: Felix Ernst

.gitlab-ci.yml

@@ -1,7 +1,6 @@
 include:
   - project: sysadmin/ci-utilities
     file:
-      - /gitlab-templates/linux-qt6.yml
       - /gitlab-templates/linux-qt6-next.yml
       - /gitlab-templates/freebsd-qt6.yml
       - /gitlab-templates/windows-qt6.yml