RFR: 8304657: G1: Rename set_state_empty to set_state_untracked

[Albert Mingkun Yang]

Trivial renaming.

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Commit messages:
 - g1-remset-untracked

Changes: https://git.openjdk.org/jdk/pull/13118/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13118&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304657
  Stats: 5 lines in 4 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/13118.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13118/head:pull/13118

PR: https://git.openjdk.org/jdk/pull/13118