RFR: 8292347: Remove unused Type::is_ptr_to_boxing_obj

[Tobias Hartmann]

`Type::is_ptr_to_boxing_obj` is unused and should be removed.

Thanks,
Tobias

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Commit messages:
 - 8292347: Remove unused Type::is_ptr_to_boxing_obj

Changes: https://git.openjdk.org/jdk/pull/9873/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9873&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292347
  Stats: 9 lines in 1 file changed: 0 ins; 9 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9873.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9873/head:pull/9873

PR: https://git.openjdk.org/jdk/pull/9873