RFR: 8220411: Remove ScavengeRootsInCode=0 code

[Stefan Karlsson]

Hi all,

Please review this patch to remove code that checks and acts upon 
ScavengeRootsInCode=0.

http://cr.openjdk.java.net/~stefank/8220411/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8220411

Since a while now, we have not supported -XX:ScavengeRootsInCode=0, and 
the code actively sets it to 1 if the user passes in 0. This patch 
removes the actual code, there's another RFE to deal with changing the 
argument handling to explicitly not support that the flag is set to 0. 
See: JDK-8144954.

My main motivation for this patch is to get rid of some of the usages of 
CollectedHeap::is_scavengable. The intention is that later patches will 
completely remove that function.

Thanks,
StefanK