Remove unused skeleton loader, css and fix mock data for roadmap code

What does this MR do?

For roadmap_shell.vue and its spec

• Removes the deprecated skeleton loader from roadmap_shell.vue (skeleton loader is no longer needed in this component as we've been using gitlab-ui's loading icon in roadmap_app.vue)

• Removes the associated skeleton loader css

• Removes .roadmap-shell and replaces it with a set of utility classes.

• Updates the spec to use the vue test utils instead of the outdated test helper.

For epic_item_timeline.vue

• Replaces .epic-bar-wrapper with gl-relative utility class in epic_item_timeline.vue.

Other changes

• Updates the mock data used in roadmap specs (left a separate comment in the discussion)

Screenshots (strongly suggested)

No visual change

Before	After

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• [-] Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content