RFR: 8298090: Use String.join() instead of manual loop in DescriptorSupport.toString

[Andrey Turbanov]

There is opportunity to use String.join in DescriptorSupport.toString implementation. Result code is much shorter and clearer.

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Commit messages:
 - [PATCH] Use String.join() instead of manual loop in DescriptorSupport

Changes: https://git.openjdk.org/jdk/pull/10960/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10960&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298090
  Stats: 9 lines in 1 file changed: 0 ins; 7 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/10960.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10960/head:pull/10960

PR: https://git.openjdk.org/jdk/pull/10960