RFR: 7903849: Scrape off Apache Ant remnants from APIDiff

[Pavel Rappo]

Apache Ant build described in README.md does not work. It seems out of date and out of sync with GNU make build. It should be removed.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/apidiff/pull/18/files
  Webrev: https://webrevs.openjdk.org/?repo=apidiff&pr=18&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903849
  Stats: 273 lines in 5 files changed: 0 ins; 264 del; 9 mod
  Patch: https://git.openjdk.org/apidiff/pull/18.diff
  Fetch: git fetch https://git.openjdk.org/apidiff.git pull/18/head:pull/18

PR: https://git.openjdk.org/apidiff/pull/18