RFR(S): 8244658: Remove dead code in code cache sweeper

[Nils Eliasson]

Hi,

Please review this removal of dead code in the sweeper.

Some are leftovers from the transition to handshakes:
- class VM_MarkActiveNMethods
- class NMethodMarkingTask
- NMethodSweeper::mark_active_nmethods

And some are just dead:
- NMethodSweeper::report_events

Bug: https://bugs.openjdk.java.net/browse/JDK-8244658
Webrev: http://cr.openjdk.java.net/~neliasso/8244658/webrev.01/

Best regards,
Nils Eliasson