RFR: 6917: Clean warnings in the agent project

[Marcus Hirt]

Also project compiler setting mismatch.

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Commit messages:
 - 6917: Clean warnings in the agent project

Changes: https://git.openjdk.java.net/jmc/pull/117/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=117&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6917
  Stats: 21 lines in 5 files changed: 9 ins; 6 del; 6 mod
  Patch: https://git.openjdk.java.net/jmc/pull/117.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/117/head:pull/117

PR: https://git.openjdk.java.net/jmc/pull/117