RFR: 8371197: G1: Use void for return type of G1RegionsOnNodes::add

[Albert Mingkun Yang]

Remove unused returns and update return-type.

Test: tier1-3

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Commit messages:
 - g1-void-return

Changes: https://git.openjdk.org/jdk/pull/28126/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28126&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371197
  Stats: 11 lines in 5 files changed: 0 ins; 3 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/28126.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28126/head:pull/28126

PR: https://git.openjdk.org/jdk/pull/28126