RFR (XXS): JDK-8022051: G1: Remove some unused G1 flags

[Bengt Rutisson]

Hi everyone,

Could I have a couple of reviews of this really small change?

http://cr.openjdk.java.net/~brutisso/8022051/webrev.00/

It removes four develop flags that are not being used anywhere in the code.

(BTW, I promise to stop looking at the flag code now. I know I've sent 
out three small flag changes in a short time. ;) )

Bengt