RFR: 8336401: Remove the option onjcmd from the jdwp agent

[Johannes Bechberger]

Remove the support for on-demand debugging via the onjcmd option.

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Commit messages:
 - Remove added empty line
 - Fix AgentEvent test
 - Remove onjcmd

Changes: https://git.openjdk.org/jdk/pull/21387/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21387&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336401
  Stats: 241 lines in 5 files changed: 0 ins; 239 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21387.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21387/head:pull/21387

PR: https://git.openjdk.org/jdk/pull/21387