[code-reflection] Integrated: Simplify pom.xml and maven build doc

[Gary Frost]

Simplified root pom.xml (no github vars needed) + maven build docs.

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Commit messages:
 - Simplify pom.xml and maven build doc

Changes: https://git.openjdk.org/babylon/pull/280/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=280&range=00
  Stats: 24 lines in 2 files changed: 0 ins; 21 del; 3 mod
  Patch: https://git.openjdk.org/babylon/pull/280.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/280/head:pull/280

PR: https://git.openjdk.org/babylon/pull/280