RFR: 8347563: C2: clean up ModRefBarrierSetC2

[Thomas Schatzl]

On Tue, 14 Jan 2025 17:47:59 GMT, Roberto Castañeda Lozano <rcastanedalo at openjdk.org> wrote:

> 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.

lgtm.

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Marked as reviewed by tschatzl (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/23112#pullrequestreview-2562166129