RFR: 8307518: Remove G1 workaround in jstat about zero sized generation sized

[Thomas Schatzl]

Hi all,

  please review removal of some workaround in g1 memory usage monitoring that made sure that there were no 0-sized generations in the output. After [JDK-8307428](https://bugs.openjdk.org/browse/JDK-8307428) this is not necessary any more.

Testing: tier1-5

Thanks,
  Thomas

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Commit messages:
 - remove obsolete comment
 - additional changes
 - initial version

Changes: https://git.openjdk.org/jdk/pull/13880/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13880&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307518
  Stats: 38 lines in 2 files changed: 0 ins; 23 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/13880.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13880/head:pull/13880

PR: https://git.openjdk.org/jdk/pull/13880