RFR: 8303151: DCmd framework cleanups

[Yi Yang]

On Fri, 3 Mar 2023 04:59:44 GMT, David Holmes <dholmes at openjdk.org> wrote:

> Whilst working on the DCmd code I noticed two items that could be cleaned up:
> 
> 1. The `NMTDCmd` is registered after the call to `register_dcmds()` instead of inside it.
> 
> 2. The "extension" mechanism to define external DCmds (as added by [JDK-7132515](https://bugs.openjdk.org/browse/JDK-7132515) for `UnlockCommercialFeatures`) is no longer needed.
> 
> Testing: tiers 1-3
> 
> Thanks

Is it possible to remove DCmdRegistrant too? Maybe `void DCmdFactory::register_dcmds` or `void register_dcmds`

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PR: https://git.openjdk.org/jdk/pull/12847