RFR: 8189734: Cleanup MarkSweep when G1 dependency is gone

[Stefan Johansson]

Hi,

Please review this change for RFE:
https://bugs.openjdk.java.net/browse/JDK-8189734

Webrev:
http://cr.openjdk.java.net/~sjohanss/8189734/00/

Summary:
This is change is based on the changes made for JEP 307 (Parallel Full 
GC for G1) and will, if reviewed, be pushed as soon as the Full GC has 
been integrated. The work in JEP 307 removes the dependency G1 currently 
have on the MarkSweep code. This changes removes the extra code added to 
the serial full GC to handle G1 heaps.

Testing:
* Local jtreg tests hotspot_gc with and without -XX:+UseSerialGC
* Built through JPRT

Thanks,
Stefan