RFR: 8347563: C2: clean up ModRefBarrierSetC2

[Roberto Castañeda Lozano]

This cleanup removes `ModRefBarrierSetC2::pre_barrier()` (which is a no-op after [JDK-8334060](https://bugs.openjdk.org/browse/JDK-8334060)) and unused arguments of `ModRefBarrierSetC2::post_barrier()`. Thanks to @TobiHartmann for reporting!

**Testing:** tier1-3.

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Commit messages:
 - Update copyright year
 - Remove unused import
 - Remove unused arguments from ModRefBarrierSetC2::post_barrier
 - Remove unused import
 - Remove no-op function ModRefBarrierSetC2::pre_barrier

Changes: https://git.openjdk.org/jdk/pull/23112/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23112&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347563
  Stats: 62 lines in 4 files changed: 0 ins; 54 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/23112.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23112/head:pull/23112

PR: https://git.openjdk.org/jdk/pull/23112