RFR: 7769: Cleanup pom and project files for JMC core [v4]

[Christoph Langer]

> There is a lot of redundancy, whitespace errors, inconsistencies, obsolete stuff, errors, outdated plugin versions etc. in JMC core pom and project files.
> 
> org.openjdk.jmc.flightrecorder.writer.test and org.openjdk.jmc.jdp.test have been refactored in terms of layout like the other test projects.

Christoph Langer has updated the pull request incrementally with one additional commit since the last revision:

  Fix merge error

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

Changes:
  - all: https://git.openjdk.java.net/jmc/pull/398/files
  - new: https://git.openjdk.java.net/jmc/pull/398/files/cc62e344..ebc8b7ba

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jmc&pr=398&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jmc&pr=398&range=02-03

  Stats: 18 lines in 1 file changed: 0 ins; 18 del; 0 mod
  Patch: https://git.openjdk.java.net/jmc/pull/398.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/398/head:pull/398

PR: https://git.openjdk.java.net/jmc/pull/398