Remove unnecessary expand sha button

What does this MR do?

This MR removes an unnecessary and buggy commit SHA expand button. See the issue for the reasons why this button is better removed than kept #25145 (closed).

This MR also cleans up some things:

• Moves details_behavior.js to a module in commit_info so that it's not loaded on every page.
• Removes unused styles.

Screenshots (strongly suggested)

Before (on master and .com)	After (this MR)

The expand button still works in the commit info box 🎉

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

References

• Related to #25145 (closed)