RFR: 8260212: Shenandoah: resolve-only UpdateRefsMode is not used

[Aleksey Shipilev]

The only "use" is `ShenandoahMarkResolveRefsClosure`, which is unused itself.

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Commit messages:
 - 8260212: Shenandoah: resolve-only UpdateRefsMode is not used

Changes: https://git.openjdk.java.net/jdk/pull/2177/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2177&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260212
  Stats: 18 lines in 2 files changed: 0 ins; 18 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2177.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2177/head:pull/2177

PR: https://git.openjdk.java.net/jdk/pull/2177