RFR (M): 8215297: Remove ParallelTaskTerminator

[Thomas Schatzl]

Hi all,

   can I have reviews for this change that removes the 
ParallelTaskTerminator code? In JDK12 we introduced another 
implementation, the OWSTTaskTerminator, that seems to work just fine as 
default for more than a year now, so I think it is time to remove the 
old implementation.

@Shenandoah-Team: I left the ShenandoahTaskTerminator wrapper, but 
removing the TaskTerminator wrapper for the other GCs. I can remove that 
too, but did not know if you wanted to keep the name.

In a follow-up change I would like to change the name of the 
OWSTTaskTerminator to just TaskTerminator (also renaming the files, but 
keeping it separate to taskqueue.?pp). Tell me if there are any concerns.

CR:
https://bugs.openjdk.java.net/browse/JDK-8215297
Webrev:
http://cr.openjdk.java.net/~tschatzl/8215297/webrev/
Testing:
hs-tier1-5, builds with Shenandoah

Thanks,
   Thomas