RFR (XS): 8235996: Move obsolete flag G1RSetScanBlockSize in flags list

[Thomas Schatzl]

Hi all,

   can I have reviews for this change that removes the now obsolete flag 
G1RSetScanBlockSize in the code, and move it down the flag lists in 
arguments.cpp?

CR:
https://bugs.openjdk.java.net/browse/JDK-8235996
Webrev:
http://cr.openjdk.java.net/~tschatzl/8235996/webrev/
Testing:
local compilation; there is no use of this flag anywhere in Hotspot code 
- verified implicitly by our CI not giving any additional failure 
because of this even before this change.

Thanks,
   Thomas