chore(waterdata): drop benchmark scaffolding from PR

The ``benchmarks/`` directory was useful during development for
measuring the hash-ID-drop impact, but it's not part of the runtime
or test surface and the numbers are captured in the PR description.
Removing to keep the diff focused on the library change.

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

Author: thodson-usgs