RFR (S): JDK-8077841: G1: Remove PrintReachable support

[Bengt Rutisson]

Hi everyone,

Could I have a couple of reviews for this change to remove the 
PrintReachable support in G1?

https://bugs.openjdk.java.net/browse/JDK-8077841
http://cr.openjdk.java.net/~brutisso/8077841/webrev.00/

There are three develop flags that allow G1 to dump all objects on the 
heap to a text format:

G1PrintReachableAtInitialMark
G1VerifyDuringGCPrintReachable
G1PrintReachableBaseFile

Turning on this dumping code will dump all objects in all regions to the 
file specified by G1PrintReachableBaseFile,

Several things are disturbing with this code:
- The way the code opens a separate file and writes to it is pretty 
uncommon for our logging code
- The flags are develop flags, so not usable in product code - even 
druring verification
- The format the heap is dumped in is not a standard format so there is 
no tool support for it
- There is already standard ways of dumping the heap, using hprof for 
example.
- It is pretty unclear when the information logged by PrintReachable is 
useful.


I've asked around a bit and as far as I can tell no one has been using 
this feature for a long time. So, rather than trying to fix the code I 
propose that we remove it.

Thanks,
Bengt