RFR: 8257708: Remove redundant unmodifiableSet wrapper from already immutable set returned by Collections.singleton

[Turbanov Andrey]

8257708: Remove redundant unmodifiableSet wrapper from already immutable set returned by Collections.singleton

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Commit messages:
 - 8257708: Remove redundant unmodifiableSet wrapper from already immutable set returned by Collections.singleton
 - [PATCH] Remove redundant unmodifiableSet wrapper from already immutable set returned by Collections.singleton

Changes: https://git.openjdk.java.net/jdk/pull/1505/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1505&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257708
  Stats: 26 lines in 2 files changed: 0 ins; 12 del; 14 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1505.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1505/head:pull/1505

PR: https://git.openjdk.java.net/jdk/pull/1505