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

[Aleksey Shipilev]

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`

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Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/5378/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5378&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273378
  Stats: 9 lines in 1 file changed: 6 ins; 3 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5378.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5378/head:pull/5378

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