Unused CSS part exists in mobile CSS framework

What does this MR do?

Removes unused CSS part in the mobile CSS framework.

What are the relevant issue numbers?

#50879 (closed)

Does this MR meet the acceptance criteria?

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

Closes #50879 (closed)