RFR: 8255721: Remove no-op clean_weak_method_links methods

[Claes Redestad]

ProfileData:: and DataLayout::clean_weak_method_links are both virtual but empty methods with no overrides, so removing them and simplifying MethodData::clean_weak_method_links should be a minor clean-up and possibly speed-up redefinition a notch.

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Commit messages:
 - Clean-up no-op clean_weak_method_links

Changes: https://git.openjdk.java.net/jdk/pull/990/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=990&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255721
  Stats: 28 lines in 2 files changed: 0 ins; 28 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/990.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/990/head:pull/990

PR: https://git.openjdk.java.net/jdk/pull/990