requirements-test: Remove swh.lister dependency

The swh.lister package was required as testing dependency to check registration of celery tasks for listers through plugins declared in swh.workers entrypoints.

However, it is easy to create a fake lister in tests data to check its correct registration through scheduler CLI so that swh.lister dependency is not really needed.

As a consequence, worker plugins are now discovered in the function register_task_types from the swh.scheduler.cli.task_type module, previoulsy it was done when importing the module.