Breaking: Remove project specific custom fields 

Description 

EXT:academic_bite_jobs provides flexform options and filters based on custom project specific fields and options not interchangeable with other projects and not making sense being provided by default.

These custom fields needs to be setup the same way on the external bite-job system and is mosts likely not the way to go.

Mangling around with the current implementation even if only the options are removed is not suitable and the internal implementation needs to be streamlined first before adding customization endpoints in a reasonable way as public API.

Based on aforementioned background project specific implementation and options are now removed to clean up the code base as preparation to streamline first the internal implementation with dedicated changes, re-introducing the feature with a new API at a later point.

This is technically breaking, at least for the one specific project and is handled as breaking for it, albeit it should not target any other project at all.

Reducing the feature set to the bare minimum simply displaying only the single job detail page without related jobs is the mostly used base ground and the essential base feature to start fresh from.

Additionally, fluid template structure is streamlined in the same step to follow the restructure epic for all academic extensions.

Impact 

Following removals are included:

  • flexform options settings.jobs.custom.zuordnung and

settings.jobs.groupBy are removed. `BiteFieldsHelper` is removed being unused and an empty class anyway. BiteJobsService->fetchBiteJobs() handling code for removed flexform setting options is removed from method. `public BiteJobsService->findCustomJobRelations()` is removed. public BiteJobsService->findCustomBiteFieldLabelsFromOptions() is removed. `public BiteJobsService->groupByRelations()` is removed. public BiteJobsService->mapFieldsToJobs() is removed.

Affected Installations 

EXT:academic_bite_jobs before v2.1 installations using custom project specific fields and options

Migration 

[TODO] How to migrate the breaking change?