[jdk11u-dev] RFR: 8210022: remove jdk.testlibrary.ProcessThread, TestThread and XRun

[Goetz Lindenmaier]

Backport this cleanup to simplify later backports.

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Commit messages:
 - Backport 370abe471ffe0385eb78128fc3d5725f7f43b544

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/877/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=877&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8210022
  Stats: 304 lines in 17 files changed: 21 ins; 266 del; 17 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/877.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/877/head:pull/877

PR: https://git.openjdk.java.net/jdk11u-dev/pull/877