RFR: 7903381: Remove temporary "Run with JUnit 4" flag from jtreg

[Christian Stein]

Please review this change removing an internal flag and unused code. Note, that the JUnit Platform still finds and runs tests using the JUnit 4 API since jtreg 7.

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Commit messages:
 - 7903381: Remove temporary "Run with JUnit 4" flag from jtreg

Changes: https://git.openjdk.org/jtreg/pull/281/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=281&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903381
  Stats: 30 lines in 1 file changed: 0 ins; 29 del; 1 mod
  Patch: https://git.openjdk.org/jtreg/pull/281.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/281/head:pull/281

PR: https://git.openjdk.org/jtreg/pull/281