docs(demos): remove unreferenced peak_discharge_trends.csv

Commit ad4e980 switched the peak-streamflow-trends demo to pull data
live and deleted the `demos/datasets/` copy of this cached CSV, but a
second copy under docs/ was missed. It is referenced nowhere in the repo,
so remove it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: thodson-usgs