Remove unused frontend variables from environments page

What does this MR do?

These were once used to display help and upgrade notices for deploy boards, but have been unused since deploy boards were moved to core in #212320 (closed).

Screenshots (strongly suggested)

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

Closes #297659 (closed)