RFR(s): 8076231: Remove unused is_in_partial_collection()

[Per Liden]

Hi,

Please review this small cleanup.

All collectors provide an implementation for is_in_partial_collection(), 
but it's actually only ever used by GenCollectedHeap. This patch removes 
the unused versions and removes the function from the CollectedHeap 
interface.

Webrev: http://cr.openjdk.java.net/~pliden/8076231/webrev.0/

Bug: https://bugs.openjdk.java.net/browse/JDK-8076231

Testing: jprt

/Per