RFR: 8375561: Class NGGroup is in need of some cleanup

[Christopher Schnick]

This should improve the code quality of the class while preserving its original workings

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Commit messages:
 - Clean up NGGroup

Changes: https://git.openjdk.org/jfx/pull/2043/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2043&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375561
  Stats: 41 lines in 1 file changed: 0 ins; 25 del; 16 mod
  Patch: https://git.openjdk.org/jfx/pull/2043.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2043/head:pull/2043

PR: https://git.openjdk.org/jfx/pull/2043