JFR oldobject tests are not run when GCs are specified explicitly

The tests are tagged with @requires == "null", which has the effect that if you run with GC explicitly set, like:
make -C ../build/fastdebug test TEST=test/jdk/jdk/jfr/event/oldobject/TestZ.java JTREG="JAVA_OPTIONS=-XX:+UseG1GC"

then the tests won't run. The intention has probably been to only run the test with the default GC. I propose that we remove this and run the tests with other GCs as well.