RFR: 8320907: Shenandoah: Remove ShenandoahSelfFixing flag

[Aleksey Shipilev]

On Tue, 28 Nov 2023 15:40:24 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> We added `ShenandoahSelfFixing` flag during LRB work to exercise LRB mid-paths better during stress tests. Today, we do not need the flag. Actually, if we ever call LRB for the side-effect of updating the location (e.g. somewhere in roots), then disabling this flag would introduce more bugs. Therefore, we need to remove the flag and make self-fixing unconditional. 
> 
> Additional testing:
>  - [x] MacOS AArch64 server fastdebug, `hotspot_gc_shenandoah`

Thank you all!

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PR Comment: https://git.openjdk.org/jdk/pull/16855#issuecomment-1832378026