Remove or rename SimpletestUiPrinter

Problem/Motivation

Simpletest and its UI is no longer used. However we still have a class at \Drupal\Tests\Listeners\SimpletestUiPrinter that turns URLs into clickable links.

Steps to reproduce

Proposed resolution

Rename or refactor the class if it's still useful, or remove it if it's not needed.

Remaining tasks

Figure out if we still need this.

User interface changes

API changes

Data model changes

Release notes snippet