Remove IIFEs from stat_graph_contributors_graph.js

What does this MR do?

Replaces the Immediately Invoked Function Expressions from javascript in the contributors stat graphing feature as requested in &2074 (closed) -> #34613 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides

Availability and Testing

• Tested in all supported browsers