RFR(M): 8155634: Clean out old logging and dead code from SurvRateGroup

[Mikael Gerdin]

Hi,

Please review this change to clean out unused code from SurvRateGroup. 
The G1YoungSurvRateNumRegionsSummary flag is only accessible in 
notproduct builds and it is my opinion that the ability to print the 
predicted aggregate survivor rates for eden regions at JVM shutdown is 
not a particularly useful feature.

Testing: Local testing with gc test suite.
Bug: https://bugs.openjdk.java.net/browse/JDK-8155634
Webrev: http://cr.openjdk.java.net/~mgerdin/8155634/webrev.0/

/Mikael