Revisit internal/ct/requestlog.go: keep, refactor, or drop

[patflynn]

Spun out of review feedback on #841 (klog → slog migration).

internal/ct/requestlog.go carries its own ad-hoc structured-logging convention (the RL: … prefix on each line) that predates the move to slog. The slog migration preserved the existing prefix but didn't take a position on whether this layer is still needed:

• The interface logs request-scoped events (origin, addDERToChain, addCertToChain, issueSCT, status) via the DefaultRequestLog impl, all at slog.LevelDebug.
• Most of what it captures arguably belongs in OpenTelemetry spans/events on the request, not in a separate per-request log channel.

Action: decide whether to

1. Drop requestLog entirely and move the per-request data onto the existing tracing spans, or
2. Keep it but rename/restructure away from the RL: prefix into proper slog attributes, or
3. Leave as-is (current state after Switch all klog to slog #841).

Probably worth doing alongside, or after, the per-request-debug logging review in the sibling issue.