RFR: 8356289: Shenandoah: Clean up SATB barrier runtime entry points

[Cesar Soares Lucas]

The runtime entry points for the SATB slow-paths currently take a JavaThread* argument. When @rkennke did the entry points for Graal ([JDK-8356075](https://bugs.openjdk.org/browse/JDK-8356075)), he figured that we do not really need them. The slow-path is only called rarely (whenever the local SATB buffer is full), and getting the current Thread* in the runtime would not be the expensive part.

This PR is a patch to do that clean up. The changes were tested on Linux x64/aarch64 with JTREG tier1-3 using Shenandoah for all tests.

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Commit messages:
 - Shenandoah API clean-up.

Changes: https://git.openjdk.org/jdk/pull/26850/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26850&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356289
  Stats: 31 lines in 9 files changed: 1 ins; 8 del; 22 mod
  Patch: https://git.openjdk.org/jdk/pull/26850.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26850/head:pull/26850

PR: https://git.openjdk.org/jdk/pull/26850