RFR (XXS): 8252086: G1: Remove g1_rs in G1CollectedHeap::initialize

[Thomas Schatzl]

Hi,

   can I have reviews for this small change that removes an intermediate 
local variable from G1CollectedHeap::initialize?

This makes it easier to follow that CollectedHeap::_reserved is the same 
as HeapRegionManager::reserved().

CR:
https://bugs.openjdk.java.net/browse/JDK-8252086
Webrev:
http://cr.openjdk.java.net/~tschatzl/8252086/webrev/
Testing:
local compilation, local jtreg gc/g1 directory

Thanks,
   Thomas