RFR (S): 8202780: Remove EnqueueTask related code from ReferenceProcessor after JDK-8202017

[Thomas Schatzl]

Hi all,

  in JDK-8202017 we removed the separate enqueue references phase for
all collectors; can I have reviews to remove some support code that is
now dead code?

CR:
https://bugs.openjdk.java.net/browse/JDK-8202780
Webrev:
http://cr.openjdk.java.net/~tschatzl/8202780/webrev/
Testing:
local compilation

Thanks,
  Thomas