fix(deps): remove obsolete pdfminer (Py2 legacy) — keep pdfminer.six only

The two packages both install to /pdfminer/, so pdfminer (last release 2019,
Python 2 era) shadows pdfminer.six (the maintained successor). This breaks
pdfplumber with:

  ImportError: cannot import name 'PDFStackT' from 'pdfminer.pdfinterp'

pdfminer.six is the correct dependency for all modern PDF tooling
(pdfplumber, markitdown, etc.). The legacy pdfminer entry was a bug from
the initial commit.

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Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

Author: 3 people

docker/requirements/python-documents.txt

@@ -5,7 +5,6 @@ PyPDF2>=3.0
 pypdf>=4.0
 pdfplumber>=0.10
 pdfminer.six>=20221105
-pdfminer>=20191125
 pdf2image>=1.16
 reportlab>=4.0