Old opencensus dependency brings in security warnings

Trillian depends on opencensus for tracing. Opencensus is deprecated, and hasn't been updated in at least 3 years: https://github.com/census-instrumentation/opencensus-go.

This dependency:
- brings in AWS SDK v1, which further led to https://github.com/google/certificate-transparency-go/issues/1673
- bring in `github.com/prometheus/prometheus`, together with `GO-2026-5264`, `GO-2026-5381` and `GO-2026-5662`

Given that Trillian is in maintenance mode, and that it's not clear who uses tracing, we'll remove tracing altogether from Trillian, hence dropping the problematic dependencies.