RFR: 8374208: Remove superfluous field in XYChart

[Andy Goryachev]

This change removes an unnecessary field `XYChart.Data.series` as it is mirrored by the `seriesProperty`, something I failed to see twice - first during code review [0] I did not realize a property was added that mirrors the field, then I wrongly thought it was added in [1].

Now is the time to gather stones.

[0] https://git.openjdk.org/jfx/pull/1016

[1] https://git.openjdk.org/jfx/pull/2013

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jfx/pull/2018/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2018&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374208
  Stats: 15 lines in 1 file changed: 8 ins; 2 del; 5 mod
  Patch: https://git.openjdk.org/jfx/pull/2018.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2018/head:pull/2018

PR: https://git.openjdk.org/jfx/pull/2018