RFR: 8320907: Shenandoah: Remove ShenandoahSelfFixing flag

[Aleksey Shipilev]

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`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/16855/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16855&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320907
  Stats: 7 lines in 3 files changed: 0 ins; 5 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/16855.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16855/head:pull/16855

PR: https://git.openjdk.org/jdk/pull/16855