RFR (S) 8252695: Shenandoah: clean up Graal enabled/disabled filter in tests

[Aleksey Shipilev]

Testbug RFE:
   https://bugs.openjdk.java.net/browse/JDK-8252695

In gc/shenandoah jtreg tests, there are !vm.graal.enabled clauses in some @requires, but not in the 
other. Since JDK 15 and JDK-8249000, we don't need the explicit Graal check in those tests.

But, for consistency within the tests and consistency across the JDK releases, we might consider 
checking for Graal explicitly. This would enable us to transplant the tests from 15u to 11u without 
remembering to check for Graal additionally.

Fix:
   https://cr.openjdk.java.net/~shade/8252695/webrev.01/

Testing: hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey