RFR: 8294695: Remove redundant deprecation suppression in ThreadGroup

[Andrey Turbanov]

This suppression were added with Loom integration, but method `Thread.getThreadGroup()` is not deprecated.
Let's cleanup code a bit.
https://mail.openjdk.org/pipermail/core-libs-dev/2022-September/094907.html

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Commit messages:
 - [PATCH] Remove redundant deprecation suppression in ThreadGroup

Changes: https://git.openjdk.org/jdk/pull/10521/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10521&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294695
  Stats: 3 lines in 1 file changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10521.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10521/head:pull/10521

PR: https://git.openjdk.org/jdk/pull/10521