RFR (XS) 8241985: Shenandoah: simplify collectable garbage logging

[Roman Kennke]

Looks good!

Thanks,
Roman


> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8241985
> 
> Webrev:
>   https://cr.openjdk.java.net/~shade/8241985/webrev.01/
> 
> See the log examples in the issue itself.
> 
> Testing: hotspot_gc_shenandoah, eyeballing gc logs
>