chore: retire Google Sheets analytics report notebooks

[NoopDog]

Part 2 of retiring the legacy analytics report formats (see #4910 for part 1 and the dependency-burden rationale: gspread/gspread-formatting and the notebook toolchain exist in analytics/requirements.txt only for these flows).

What to delete

• analytics/example/ — the entire directory (example sheets report: constants.py, generate_sheets_report.ipynb, readme.md, users_over_time_history.json).
• analytics/anvil-catalog-sheets/generate_sheets_report.ipynb
• analytics/anvil-explorer-sheets/generate_sheets_report.ipynb — KEEP utils.py (used by generate_static_site.py for dataset-title resolution).
• analytics/hca-explorer-sheets/generate_sheets_report.ipynb
• analytics/lungmap-analytics/sheets/generate_sheets_report.ipynb
• In each surviving constants.py (anvil-catalog-sheets, anvil-explorer-sheets, hca-explorer-sheets, lungmap-analytics/sheets): remove the Sheets-only constants SHEET_NAME and PARENT_FOLDER_NAME. Verified: no generate_static_site.py imports them.
• KEEP anvil-catalog-sheets/users_over_time_history.json — its static site reads it (constants.py:14, HISTORIC_UA_DATA_PATH = "users_over_time_history.json").
• Update the per-directory readme.md files to drop the "Generating Reports" notebook instructions, leaving the static-site instructions.

Verification

Run each of the four generate_static_site.py scripts — all must complete with unchanged output (the constants removal must not break imports).

Order

Independent of #4910; the package slimming (part 3) depends on both.