RFR: 8279168: Shenandoah: Remove unused always_true in ShenandoahRootAdjuster::roots_do()

[Zhengyu Gu]

A trivial patch to remove unused always_true variable.

Test:
- [x] Built on Linux x86_64

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Commit messages:
 - Merge branch 'master' into JDK-8279168-unused-always_true
 - 8279168: Shenandoah: Remove unused always_true in ShenandoahRootAdjuster::roots_do()

Changes: https://git.openjdk.java.net/jdk/pull/6917/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6917&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279168
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6917.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6917/head:pull/6917

PR: https://git.openjdk.java.net/jdk/pull/6917