Drop derivative cd_documentation_extracted xlsx

The earlier commit 853e25a migrated three CD documentation files
into tmd/areas/prepare/data/soi_cds/. Two of them
(21incddocguide.docx, 22incddocguide.docx) are SOI's original
published variable-definition documentation and are irreplaceable
without re-downloading from IRS. The third,
cd_documentation_extracted_from_21incddocguide.docx.xlsx, is a
hand-extracted spreadsheet derivative of the 2021 docx — useful
for programmatic lookup of variable definitions, but reproducible
from the docx and not original SOI source material.

Removing the derivative keeps tmd/areas/prepare/data/soi_cds/
limited to original SOI sources (data + documentation). If a
structured/extracted copy is needed in the future, it can be
regenerated from 21incddocguide.docx.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: donboyd5