RFR: 8373441: Remove DCmdFactory::_enabled

[Ioi Lam]

The `DCmdFactory::_enabled` is always set to `true` and there doesn't seem to be a reason to set it to `false`.

This PR removes this field and simplified the creation of `DCmdFactory` objects.

The related `_hidden` field is also currently not used, but may be used in the future when deprecating DCmds, so we leave it unchanged.

Note that now `jmm_GetDiagnosticCommandInfo()` always set `dcmdInfo::enabled` to `true` to be compatible with Java code.

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Commit messages:
 - Added comments about Java API
 - 8373441: Remove DCmdFactory::_enabled

Changes: https://git.openjdk.org/jdk/pull/28794/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28794&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373441
  Stats: 100 lines in 6 files changed: 4 ins; 25 del; 71 mod
  Patch: https://git.openjdk.org/jdk/pull/28794.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28794/head:pull/28794

PR: https://git.openjdk.org/jdk/pull/28794