Remove workflows

[mweidling]

This PR removes the whole workflow section from the back end since this is handled by quiver-benchmarks.

It also updates all relevant JSON files.

[kba]

LGTM

[stweil]

The QuiVer benchmarks still refer to the removed workflows.

Example workflows/results/reichsanzeiger_random_minimal_ocr_result.json:

{
    "eval_workflow_id": "wf-datareichsanzeiger_random-eval",
    "label": "Workflow on data reichsanzeiger_random",
    "metadata": {
        "ocr_workflow": {
            "@id": "https://github.com/OCR-D/quiver-back-end/blob/main/workflows/ocrd_workflows/minimal_ocr.txt",
            "label": "OCR Workflow minimal_ocr"
        },
        "eval_workflow": {
            "@id": "https://github.com/OCR-D/quiver-back-end/blob/main/workflows/ocrd_workflows/dinglehopper_eval.txt",
            "label": "Evaluation Workflow dinglehopper_eval"
        },
[...]