RFR: 8152086: Remove SpaceMangler::mangle_region logging

[Stefan Karlsson]

Hi all,

Please review this patch to remove the non-product mangling log output, 
that shows up when we run with -Xlog:gc=trace.

http://cr.openjdk.java.net/~stefank/8152086/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8152086

These lines only print a subset of all memory that we mangle. As a 
separate RFE we might want to add a zap logging tag and annotate all 
places where we zap/mangle memory.

StefanK