gitlab-ci: remove "when: always" in flatpak job.

This clashes with the usage of "rules:" and is even more useless as it
is about checking success of earlier stages (yet this job doesn't need
to wait for any other job).
Fixes CI error:

> jobs:flatpak config key may not be used with `rules`: when

.gitlab-ci.yml

@@ -552,7 +552,6 @@ flatpak:
   extends: .flatpak
   stage: gimp
   needs: []
-  when: always
   rules:
     # Custom builds though web GUI, API or schedules.
     - if: '$GIMP_CI_FLATPAK != null'