RFR: 8334293: G1: Refactor G1ConcurrentMark::update_top_at_rebuild_start

[Albert Mingkun Yang]

Simple removing an always true condition.

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Commit messages:
 - g1-rebuild-start

Changes: https://git.openjdk.org/jdk/pull/19718/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19718&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334293
  Stats: 19 lines in 3 files changed: 2 ins; 16 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/19718.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19718/head:pull/19718

PR: https://git.openjdk.org/jdk/pull/19718