RFR: CODETOOLS-7903622: Remove support for `jtdiff`

[Jonathan Gibbons]

Please review a trivial PR to remove the code for the long-unused `jtdiff` utility.

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Commit messages:
 - CODETOOLS-7903622: Remove support for `jtdiff`

Changes: https://git.openjdk.org/jtreg/pull/178/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=178&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903622
  Stats: 3322 lines in 22 files changed: 0 ins; 3321 del; 1 mod
  Patch: https://git.openjdk.org/jtreg/pull/178.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/178/head:pull/178

PR: https://git.openjdk.org/jtreg/pull/178