jmx-dev RFR: 8373441: Remove DCmdFactory::_enabled [v2]

[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.

Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:

  @kevinjwalls comments

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/28794/files
  - new: https://git.openjdk.org/jdk/pull/28794/files/8a047713..98176aa7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28794&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28794&range=00-01

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