RFR: 8299774: SYNTH_BUTTON_UI_KEY field is unused

[SWinxy]

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

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Commit messages:
 - SYNTH_BUTTON_UI_KEY field is unused

Changes: https://git.openjdk.org/jdk/pull/11890/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11890&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299774
  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11890.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11890/head:pull/11890

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