fix(demos): drop CSV cache from peak_streamflow_trends; run analysis live

The notebook had its 3-state ``peak_trend_analysis`` invocation
commented out with a "few minutes to run" warning, and the last cell
loaded a pre-computed ``datasets/peak_discharge_trends.csv`` instead.
The relative path broke whenever the notebook was executed from any
cwd other than ``demos/`` (e.g., nbconvert against a staged copy).

With the async chunker, the full 3-state ``get_peaks`` fan-out runs
end-to-end in ~20s, so the cached CSV no longer earns its keep:

- Un-commented the live ``peak_trend_analysis`` call in cell 11; updated
  the leading comment to note the chunker-driven runtime.
- Deleted the ``pd.read_csv("datasets/peak_discharge_trends.csv")`` cell
  (no longer needed; ``final_df`` is produced live).
- Deleted ``demos/datasets/peak_discharge_trends.csv`` (535 KB tracked
  cache) and the now-empty ``demos/datasets/`` directory.

The notebook now has no relative-path data dependency — it executes
cleanly from any cwd, including in nbconvert against a staged copy.
Verified live: full run completes in ~21s.

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

Author: thodson-usgs