src/clock_ntp_ota_v1.9.ino was a duplicate of firmware/clock_ntp_ota_v1.9/
that had already drifted out of sync. Removing it eliminates the need to
patch both copies on every PR.
Updated CONTRIBUTING.md build path to point to firmware/clock_ntp_ota_v1.9.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>