RFR: 8326414: Serial: Inline SerialHeap::create_rem_set

[Albert Mingkun Yang]

Trivial inlining a single-line method.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/17948/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17948&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326414
  Stats: 7 lines in 2 files changed: 0 ins; 6 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/17948.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17948/head:pull/17948

PR: https://git.openjdk.org/jdk/pull/17948