Remove JUNIT_RUN_WITH_JUNIT_4 flag and related code that was introduced temporarily with the support for JUnit Platform in jtreg 7.
https://github.com/openjdk/jtreg/blob/1ad8188f7788b1282302649c344d94e20bd3c42a/src/share/classes/com/sun/javatest/regtest/agent/JUnitRunner.java#L47-L48
https://github.com/openjdk/jtreg/blob/1ad8188f7788b1282302649c344d94e20bd3c42a/src/share/classes/com/sun/javatest/regtest/agent/JUnitRunner.java#L47-L48