RFR: 8273378: Shenandoah: Remove the remaining uses of os::is_MP

[Aleksey Shipilev]

On Mon, 6 Sep 2021 11:03:43 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> JDK-8188764 removed many uses of `os::is_MP`, effectively defaulting it to `true`, but some Shenandoah code still has it. This is a simple omission. All current uses on x86 already imply lock prefix, so this is a cleanup, and not a functional change.
> 
> Additional testing:
>  - [x] Linux x86_64 `hotspot_gc_shenandoah`

Thank you, GHA tests are green, so I am integrating.

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PR: https://git.openjdk.java.net/jdk/pull/5378