RFR: 8284581: Serial: Remove unused GenCollectedHeap::collect_locked

[Albert Mingkun Yang]

Trivial change of removing some dead code.

Test: build

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Commit messages:
 - trivial

Changes: https://git.openjdk.java.net/jdk/pull/8163/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8163&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284581
  Stats: 9 lines in 2 files changed: 0 ins; 9 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8163.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8163/head:pull/8163

PR: https://git.openjdk.java.net/jdk/pull/8163