RFR: JDK-8193780: (ref) Remove an undocumented "jdk.lang.ref.disableClearBeforeEnqueue" system property

[mandy chung]

Webrev:
http://cr.openjdk.java.net/~mchung/jdk11/webrevs/8193780/webrev.00/
CSR:
  https://bugs.openjdk.java.net/browse/JDK-8193781

This patch removes the "jdk.lang.ref.disableClearBeforeEnqueue" system 
property support which was added to provide an interim solution to 
restore to JDK 8 behavior of `Reference::enqueue` not to clear the 
referent.  I propose to remove this in 11.

Mandy