chore(scripts): delete unreferenced helper scripts

None of these is referenced by an installer, systemd unit, CI workflow,
test, the web UI or src/:

- utils/cleanup_venv.sh removes venv_web_v2, which nothing creates
- utils/clear_python_cache.sh hardcodes ~/LEDMatrix and a .webassets-cache
  nothing uses
- install/migrate_config.sh only copies the template, which the installer
  and ConfigManager already do
- install/debug_install.sh, debug/debug_web_manual.py
- diagnose_web_ui.sh and verify_web_ui.sh overlap diagnose_web_interface.sh,
  which the docs point to
- fix_internet_connectivity.sh is iptables-only (stale on nftables)
- diagnose_plugin_permissions.sh, dev/validate_python.py
- download_nba_logos.py + README_NBA_LOGOS.md: logo_downloader fetches
  logos on demand
- setup_plugin_repos.py linked into the production plugin-repos/ dir; the
  dev workflow is scripts/dev/dev_plugin_setup.sh, and
  MULTI_ROOT_WORKSPACE_SETUP.md now uses it

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/MIGRATION_GUIDE.md

@@ -19,7 +19,6 @@ All installation scripts have been moved from the project root to `scripts/insta
 | `install_wifi_monitor.sh` | `scripts/install/install_wifi_monitor.sh` |
 | `setup_cache.sh` | `scripts/install/setup_cache.sh` |
 | `configure_web_sudo.sh` | `scripts/install/configure_web_sudo.sh` |
-| `migrate_config.sh` | `scripts/install/migrate_config.sh` |
 
 #### Permission Fix Scripts