Allow Issue and Merge Request sidebar to be toggled from collapsed state

What does this MR do?

Remove obsolete usage of gl.lazyLoader.loadCheck

In [!21565][0] we removed the loadCheck function from the LazyLoader for
images. The function seemed to be obsolete before anyhow, because we
make use of the MutationObserver in order to detect elements added /
removed from the DOM.

[0]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21565

What are the relevant issue numbers?

Closes #52532 (closed)

Does this MR meet the acceptance criteria?

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