RFR: 8260414: Remove unused set_single_threaded_mode() method in task executor

[Leo Korinth]

This code is not used any more.

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Commit messages:
 - 8260414: Remove unused set_single_threaded_mode() method in task executor

Changes: https://git.openjdk.java.net/jdk/pull/2558/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2558&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260414
  Stats: 9 lines in 2 files changed: 0 ins; 8 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2558.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2558/head:pull/2558

PR: https://git.openjdk.java.net/jdk/pull/2558