RFR: 6744: Upgrading to Eclipse RCP 2020-03

[Marcus Hirt]

Using 2020-03 as default. Removing 2019-09.

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Commit messages:
 - 6744: Upgrading to Eclipse RCP 2020-03

Changes: https://git.openjdk.java.net/jmc/pull/69/files
 Webrev: https://webrevs.openjdk.java.net/jmc/69/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6744
  Stats: 267 lines in 9 files changed: 131 ins; 129 del; 7 mod
  Patch: https://git.openjdk.java.net/jmc/pull/69.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/69/head:pull/69

PR: https://git.openjdk.java.net/jmc/pull/69