RFR: 8329761: Remove unused KeyBuilder and unusedSets from StyleContext

[Alexey Ivanov]

Remove unused class `KeyBuilder` and unused field `unusedSets`.  
Update a comment which used javadoc syntax.  
Mark `KeyEnumeration` and `FontKey` classes as *`final`*.

All the client tests are green.

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Commit messages:
 - 8329761: Remove unused KeyBuilder and unusedSets from StyleContext

Changes: https://git.openjdk.org/jdk/pull/18659/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18659&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329761
  Stats: 156 lines in 1 file changed: 0 ins; 150 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/18659.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18659/head:pull/18659

PR: https://git.openjdk.org/jdk/pull/18659