RFR(T) : 8244384 : @requires-related clean up in gc/metaspace/ tests

[Igor Ignatyev]

http://cr.openjdk.java.net/~iignatyev//8244384/webrev.00
> 19 lines changed: 1 ins; 9 del; 9 mod; 

Hi all,

could you please review this trivial clean up in two gc/metaspace/ tests?
from JBS:
> test/hotspot/jtreg/gc/metaspace/CompressedClassSpaceSizeInJmapHeap.java and TestCapacityUntilGCWrapAround.java can be cleaned up a bit: 
>  - TestCapacityUntilGCWrapAround.java can use `@requires vm.bits` to be executed on only on suitable platforms 
>  - usage of jdk.test.lib.Platform can be removed from both tests

JBS: https://bugs.openjdk.java.net/browse/JDK-8244384
webrev: http://cr.openjdk.java.net/~iignatyev//8244384/webrev.00

Thanks,
-- Igor