RFR: 8242901: Duplicate PSYoung/OldGen max size functions

[Kim Barrett]

Please review this cleanup of ParallelGC max generation size
accessors.  There are some lingering inconsistencies left over after
the removal of UseAdaptiveGCBoundary and associated cleanups.

We had the following:

PSYoungGen: max_size(), gen_size_limit()
Their values were calculated differently but (after removal of
UseAdaptiveGCBoundary) always have the same value.

PSOldGen: max_gen_size(), gen_size_limit()
They were virtual, but that's no longer needed or useful, as there are
no longer any derived classes.  They were both returning the value of
_max_gen_size.

Both classes have min_gen_size(), an accessor for _min_gen_size.

These have been made consistent for both classes:

min_gen_size() -- as before
max_gen_size() -- accessor for _max_gen_size

There are some assertions involving max_gen_size and the reserved size
for the generations that are perhaps redundant or could be tightend
to equality tests, but I've left them as is for now.

CR:
https://bugs.openjdk.java.net/browse/JDK-8242901

Webrev:
https://cr.openjdk.java.net/~kbarrett/8242901/open.00/

Testing:
mach5 tier1-5