Fix CI now on Drupal 11.4 (CronJobInstallTest failure)

Problem/Motivation

CI moved to core 11.4, which removed FieldHooks::cron() and moved the purge pass into system_cron, see #2907780: Add a field purger service
Ultimate Cron discovers jobs from hook_cron implementations, so field_cron no longer exists and CronJobInstallTest::testManageJob() fails on its "Purge deleted Field API data" assertion.

Steps to reproduce

Run CronJobInstallTest against Drupal 11.4.

Proposed resolution

Drop the field install and its assertion. It duplicates what the test already does with file (install, assert the job appears, uninstall, assert it is gone), so no coverage is lost and no version conditional is needed.

Remaining tasks

Review.

User interface changes

None.

API changes

None.

Data model changes

None.