RFR: JDK-8076267 - Remove n_gens()

[Jesper Wilhelmsson]

Hi,

Please review this next cleanup in the backwaters of the generation array 
removal. This change removes the n_gens() method and the max_gens variable from 
GenCollectedHeap.

Next in line in the cleanup queue is a change to remove the levels concept. 
Mostly all places where we today pass a "level" in the GC code can be removed, 
or be made more explicit by introducing two constants, YOUNG and OLD. Removing 
n_gens() and the levels touches upon the same code in many places and we could 
skip one, in some places somewhat ugly, intermediate version by doing both 
changes at the same time. The levels removal is a much larger and trickier 
change though, so to make it easier to review I choose to split these into two 
changes. Please keep this in mind when reviewing the current change.

Bug: https://bugs.openjdk.java.net/browse/JDK-8076267
Webrev: http://cr.openjdk.java.net/~jwilhelm/8076267/webrev.00/

Thanks,
/Jesper