RFR: JDK-8148951: Remove unused method Generation::performs_in_place_marking()

[Bengt Rutisson]

Hi everyone,

Could I have a couple of reviews for this small cleanup?

http://cr.openjdk.java.net/~brutisso/8148951/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8148951

The methods Generation::performs_in_place_marking() is documented as 
that it should be used by biased locking. But it is currently unused and 
a "hg grep performs_in_place_marking" does not indicate that it was ever 
used.

Currently it is only used in two asserts, but these operate on a young 
or an old generation instance, where performs_in_place_marking() adds no 
value since it returns true for old generations and false for young.

Thanks,
Bengt