Remove unused skeleton loader CSS

What does this MR do and why?

This MR removes the skeleton-loader class from the roadmap.scss as this class is not defined anywhere in GitLab/GitLab UI. Thus, the CSS rule isn't doing anything.

Screenshots or screen recordings

N/A

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #363775 (closed)