RFR: 8367601: Remove held_monitor_count

[Fei Yang]

On Tue, 30 Sep 2025 09:43:51 GMT, Fredrik Bredberg <fbredberg at openjdk.org> wrote:

> Since we have removed all other locking modes than lightweight locking (see: [JDK-8344261](https://bugs.openjdk.org/browse/JDK-8344261)), we no longer need:
> - `_held_monitor_count`
> - `_parent_held_monitor_count`
> - `_jni_monitor_count`
> 
> This PR removes them from shared code as well as from `X86`, `AArch64`, `PowerPC` and `RISC-V`.
> They are not present in other platforms.
> 
> Tested tier1-7 (on supported platforms) without seeing any problems that can be traced to this code change.
> `PowerPC` and `RISC-V` has been sanity checked using QEMU.

RISC-V part of the change seems fine to me. My local hs:tier1-hs:tier3 passed with fastdebug build.

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Marked as reviewed by fyang (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/27570#pullrequestreview-3287111055