Remove slugify text utility

What does this MR do?

This removes an unused method which had outdated documentation.

How do I know this is unused?

In GitLab EE master, run the following grep:

git grep slugify -- '*.js' '*.vue'

You'll see that the only place slugify (not slugifyWithHyphens) is used is when it's defined in text_utility.js and tested in text_utility_spec.js.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content