Remove redundent logging code (#4059)

1. The `CustomFmtContext::ContextWithFormatFields` enum arm isn't
actually used and thus we don't need the enum anymore.

2. We don't do anything with most of the normalized metadata that's
created by calling `event.normalized_metadata();` - the `target` we can
get from `event.metadata.target()` and level we can get from
`event.metadata.level()` - let's just call them direct to simplify
things. (`event.metadata()` is just a field call under the hood)

Changelog: No functional changes, might run a tad faster with lots of
logging turned on.

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Co-authored-by: Bastian Köcher <[email protected]>