Remove the UI tests check for `ab-testing` PRs

[JamieB-gu]

Currently the Chromatic UI tests are a required check on PRs that only change code in the ab-testing directory. See #15431 or #15354 for examples.

This is probably causing unnecessary Chromatic runs, as this check is for testing changes to components in DCAR, which do not exist in this directory. We may want to exclude ab-testing from this check if possible.