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.

---------

Co-authored-by: Bastian Köcher <git@kchr.de>

prdoc/pr_4059.prdoc

@@ -0,0 +1,13 @@
+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: Remove redundent logging code
+
+doc:
+  - audience: Node Dev
+    description: |
+      Simplified logging code, now does slightly less work while logging.
+
+crates:
+- name: sc-tracing
+  bump: minor