Remove label-list-item references

What does this MR do and why?

Prompted from this MR review. The label-list-item CSS class exists:

1. To act as a selector for JS and tests
2. To add a border between elements

We don't need a label-specific class for this and we actually reference this label-specific class in web_hooks which has nothing to do with labels.

To clean this up we:

1. Add a js-label-list-item so that JS and tests can keep accessing these elements
2. Add a generic gl-last-of-type-border-b-0 utility class

Screenshots or screen recordings

No visual changes

Borders still work ✅

How to set up and validate locally

1. Start GDK
2. Visit a page like /gitlab-org/gitlab-shell/-/labels and make sure there's a border between each label

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.