RFR: 8280018: Remove obsolete VM_GenCollectFullConcurrent

[Albert Mingkun Yang]

Trivial change of removing dead code.

Test: build

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Commit messages:
 - trivial

Changes: https://git.openjdk.java.net/jdk/pull/7084/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7084&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280018
  Stats: 4 lines in 3 files changed: 0 ins; 4 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7084.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7084/head:pull/7084

PR: https://git.openjdk.java.net/jdk/pull/7084