RFR: 8356751: IGV: clean up redundant field _should_send_method

[Benoît Maillard]

This PR removes private field `IdealGraphPrinter::_should_send_method` (as it was only ever assigned `true`) and all usages in conditions.

### Testing

- [x] [GitHub Actions](https://github.com/benoitmaillard/jdk/actions?query=branch%3AJDK-8356751)
- [x] tier1, and some Oracle internal testing
- [x] Ran IdealGraphVisualizer and made sure that no crash occured

Shout out to @mhaessig for introducing me to IGV

Thanks!

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Commit messages:
 - 8356751: Remove redundant field _should_send_method

Changes: https://git.openjdk.org/jdk/pull/25714/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25714&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356751
  Stats: 11 lines in 2 files changed: 0 ins; 5 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/25714.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25714/head:pull/25714

PR: https://git.openjdk.org/jdk/pull/25714