RFR: 8329761: Remove unused KeyBuilder and unusedSets from StyleContext

[Sergey Bylokhov]

On Fri, 5 Apr 2024 15:46:06 GMT, Alexey Ivanov <aivanov at openjdk.org> wrote:

> 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.

Marked as reviewed by serb (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/18659#pullrequestreview-1984044967