chore(assets): drop unreferenced pptx source from the repo

sourcetextgraphics.pptx is the editable source for the diagram PNGs in
assets/images, but nothing in the repo references it and a stale
PowerPoint lock file kept leaving it dirty in the working tree.

Moved out of version control; the file itself is retained locally and
remains recoverable from history at 4bb85c7 if the diagrams ever need
re-editing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: rogeralsing