RFR: 8303151: DCmd framework cleanups

[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

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Commit messages:
 - 8303151: DCmd framework cleanups

Changes: https://git.openjdk.org/jdk/pull/12847/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12847&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303151
  Stats: 13 lines in 3 files changed: 1 ins; 11 del; 1 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