RFR: 1402: Cleanup some commands

[Guoxiong Li]

Hi all,

During the review of SKARA-1096 [1][2], I find the `AllowCommand` and `RejectCommand` are never used.
And the `OpenCommand` and `CleanCommand` have many unused import statements.
It is good to remove/cleanup them.

Thanks for taking the time to review.

Best Regards,
-- Guoxiong

[1] https://bugs.openjdk.java.net/browse/SKARA-1096
[2] https://github.com/openjdk/skara/pull/1297

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Commit messages:
 - 1402: Cleanup some commands

Changes: https://git.openjdk.java.net/skara/pull/1301/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1301&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1402
  Stats: 500 lines in 6 files changed: 0 ins; 498 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/1301.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1301/head:pull/1301

PR: https://git.openjdk.java.net/skara/pull/1301