RFR (S/M): 8231670: Remove TaskExecutor abstraction used in preserved marks processing

[Thomas Schatzl]

Hi all,

   can I get reviews for this cleanup of the preserved marks handling 
that removes the unnecessary TaskExecutor "framework". This code is 
obsolete since parallel gc also uses WorkGangs (JDK-8204951).

CR:
https://bugs.openjdk.java.net/browse/JDK-8231670
Webrev:
http://cr.openjdk.java.net/~tschatzl/8231670/webrev/
Testing:
hs-tier1-5 almost completed without issues

Thanks,
   Thomas