RFR: 8261356: Clean up enum G1Mark

Mon Feb 8 18:45:55 UTC 2021

After removing the effectively dead entry in `G1Mark`, the whole enum could be turned into a bool. The call-chain is updated and existing comments are revised.

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Commit messages:
 - bool

Changes: https://git.openjdk.java.net/jdk/pull/2461/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2461&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261356
  Stats: 29 lines in 4 files changed: 1 ins; 11 del; 17 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2461.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2461/head:pull/2461

PR: https://git.openjdk.java.net/jdk/pull/2461