Remove unused CSS class

What does this MR do and why?

Remove unused "dashboard-card-footer-failed" CSS class

The following discussion from !84098 (merged) should be addressed:

• @pslaughter started a discussion:

  polish (non-blocking): Technically we don't need this class anymore since it's background-color rule is overwritten. Not a big deal and something we could handle in a follow-up 😄

  https://gitlab.com/gitlab-org/gitlab/blob/c890f9983d9cace376baf56f99c12a78a560a9cb/app/assets/stylesheets/components/dashboard_skeleton.scss#L54

This MR removes the unused "dashboard-card-footer-failed" CSS class from the following files:

app/assets/stylesheets/components/dashboard_skeleton.scss

ee/app/assets/javascripts/vue_shared/dashboards/components/project_pipeline.vue

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #361215 (closed)