RFR: 8336401: Remove the option onjcmd from the jdwp agent [v2]

[Johannes Bechberger]

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

Johannes Bechberger has updated the pull request incrementally with one additional commit since the last revision:

  Use port 0 in TestAgentEvent

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21387/files
  - new: https://git.openjdk.org/jdk/pull/21387/files/f76f8db0..17b7b5a5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21387&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21387&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 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