RFR: 8274466 G1: simplify G1 collector states

[Hamlin Li]

This is a minor improvement which remove the redundant G1 Collector states usages.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/5745/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5745&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274466
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5745.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5745/head:pull/5745

PR: https://git.openjdk.java.net/jdk/pull/5745