RFR: 8337982: Remove dead undef assrt0n

[Casper Norrbin]

Hello everyone,

`blockTree.cpp` contains an `#undef` that is never defined anywhere.
This is dead code should be removed.

Thanks.

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Commit messages:
 - removed unnecessary undef in blockTree.cpp

Changes: https://git.openjdk.org/jdk/pull/20493/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20493&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337982
  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/20493.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20493/head:pull/20493

PR: https://git.openjdk.org/jdk/pull/20493