Fix code reuse issue in Projects::Clusters::ApplicationController#index

What does this MR do?

Fix code reuse issue in app/controllers/projects/clusters/applications_controller.rb#index

This should enable us to re-use code when we have group level clusters (for which we most probably will have new controllers)

Also remove useless inheritance from Clusters::Applications::ScheduleInstallationService which requires project to passed in.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/34758

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [-] Conforms to the database guides