RFR: 8261356: Clean up enum G1Mark [v3]

[Albert Mingkun Yang]

> 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.

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  Update src/hotspot/share/gc/g1/g1RootClosures.cpp
  
  Co-authored-by: Stefan Johansson <54407259+kstefanj at users.noreply.github.com>

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2461/files
  - new: https://git.openjdk.java.net/jdk/pull/2461/files/1a17ae19..bc69d56f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2461&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2461&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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