Remove references to `slog`

[michaelsproul]

There's a TODO added in this PR about tracing:

• Back merge release-v7.0.0.beta.4 to unstable #7194

lighthouse/common/logging/src/sse_logging_components.rs

Line 1 in cd5632c

// TODO(tracing) fix the comments below and remove reference of slog::Drain

There might be some more slog comments lurking.

Someone from SigP should go through an remove/rewrite them. We won't accept external PRs for this issue.

[chong-he]

Closed by #7378