8331683: Clean up GetCarrierThread

[alexmenkov]

JVMTI GetCarrierThread extension function was introduced by loom for testing. It's used by several tests in hotspot/jtreg/serviceability.

Testings: tier1..tier6

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Progress

• [ ] Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• [x] Change must not contain extraneous whitespace
• [x] Commit message must refer to an issue

Issue

• JDK-8331683: Clean up GetCarrierThread (Enhancement - P4)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/19289/head:pull/19289
$ git checkout pull/19289

Update a local copy of the PR:
$ git checkout pull/19289
$ git pull https://git.openjdk.org/jdk.git pull/19289/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 19289

View PR using the GUI difftool:
$ git pr show -t 19289

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/19289.diff

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