fix: remove 29 dangling entries from ground_truth_mapping.json

Remove entries referencing non-existent ground truth files that were
causing the validate_ground_truth.py check to fail and blocking the
benchmark workflow. These entries pointed to source documents that
don't exist in the repository (pdf_academic_paper, pdf_forms, etc.)
or only exist in vendored/docling without corresponding ground truth
text files.

Author: Goldziher