RFR: 8372977: unnecessary gthread-2.0 loading

[Alexander Zvegintsev]

Actual usage of the gthread-2.0 was removed with GTK2 support in [JDK-8329471](https://bugs.openjdk.org/browse/JDK-8329471).
But there are some leftovers, we are still loading it with GTK3, even though it was never used when it was added in [JDK-8164321](https://bugs.openjdk.org/browse/JDK-8164321).

It wasn't an issue before, but some modern systems (e.g. openSUSE 16) don't provide the library.
Because of this, we are unable to load GTK, which leads to Robot functionality requiring the Screencast or Remote Desktop API on Wayland not working.

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Commit messages:
 - remove GThreadFunctions
 - 8372977: unnecessary gthread-2.0 loading

Changes: https://git.openjdk.org/jdk/pull/28625/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28625&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372977
  Stats: 27 lines in 2 files changed: 0 ins; 27 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28625.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28625/head:pull/28625

PR: https://git.openjdk.org/jdk/pull/28625