RFR: 8264178: Unused method Threads::nmethods_do

[Albert Mingkun Yang]

Trivial change of removing an unused method.

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Commit messages:
 - thread

Changes: https://git.openjdk.java.net/jdk/pull/3199/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3199&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264178
  Stats: 14 lines in 2 files changed: 0 ins; 14 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3199.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3199/head:pull/3199

PR: https://git.openjdk.java.net/jdk/pull/3199