RFR: 7510: Remove 2020-06 platform

[Marcus Hirt]

Last platform for JDK 8.

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

Commit messages:
 - 7510: Remove 2020-06 platform

Changes: https://git.openjdk.java.net/jmc/pull/358/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=358&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7510
  Stats: 155 lines in 5 files changed: 0 ins; 155 del; 0 mod
  Patch: https://git.openjdk.java.net/jmc/pull/358.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/358/head:pull/358

PR: https://git.openjdk.java.net/jmc/pull/358