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

[Iris Clark]

On Mon, 1 Sep 2025 08:27:13 GMT, Christian Stein <cstein at openjdk.org> wrote:

> 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.

Marked as reviewed by iris (Reviewer).

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PR Review: https://git.openjdk.org/jtreg/pull/281#pullrequestreview-3174034133