RFR: 8299692: G1: Remove unused G1BlockOffsetTable::is_card_boundary

[Albert Mingkun Yang]

Trivial change of removing dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/11867/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11867&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299692
  Stats: 8 lines in 2 files changed: 0 ins; 8 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11867.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11867/head:pull/11867

PR: https://git.openjdk.org/jdk/pull/11867