RFR: 8303151: DCmd framework cleanups [v2]

[David Holmes]

> 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

David Holmes has updated the pull request incrementally with one additional commit since the last revision:

  Fix comment

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12847/files
  - new: https://git.openjdk.org/jdk/pull/12847/files/4e8d6b56..7f29eba9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12847&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12847&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12847.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12847/head:pull/12847

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