RFR: 8330359: G1: Remove unused forward declaration in g1BlockOffsetTable.hpp

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/18797/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18797&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330359
  Stats: 3 lines in 1 file changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18797.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18797/head:pull/18797

PR: https://git.openjdk.org/jdk/pull/18797