RFR: 8340144: C1: remove unused Compilation::_max_spills

[Denghui Dong]

Hi,

Please review this trivial change that removed the unused field Compilation::_max_spills.

Thanks

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Commit messages:
 - 8340144: C1: remove unused Compilation::_max_spills

Changes: https://git.openjdk.org/jdk/pull/21007/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21007&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340144
  Stats: 5 lines in 2 files changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/21007.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21007/head:pull/21007

PR: https://git.openjdk.org/jdk/pull/21007