RFR: 8299774: SYNTH_BUTTON_UI_KEY field is unused

[Sergey Bylokhov]

On Sat, 7 Jan 2023 03:34:02 GMT, SWinxy <duke at openjdk.org> wrote:

> Added in JDK-8192888, and became unused in JDK-8213843.

Marked as reviewed by serb (Reviewer).

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PR: https://git.openjdk.org/jdk/pull/11890